Mercari vs eBay — Which Is Better for Sellers?
A side-by-side breakdown of fees, buyer reach, returns, payouts and what real sellers say — with the maths on what you actually keep, and why Mercari is not an option outside the US.
- Choose Mercari if you want the lowest-friction listing experience in US resale — list, ship, done, with no social performance required and the platform paying return postage on approved returns.
- Choose eBay if you sell rare, collectible, branded or high-value items, or you sell outside the US at all. Mercari has no UK marketplace — it exited in 2019.
- Fees are now almost identical. On a $30 item you keep $26.25 on Mercari and $25.52 on eBay US. The fee gap every comparison article leads with is 73 cents.
- Scale is not close: eBay has 136 million active buyers. Mercari US has 4.3 million monthly active users — roughly 30 to 1.
- Returns flip the usual assumption: Mercari pays the return label and gives sellers a 72-hour auto-release. eBay makes the seller pay return shipping on not-as-described claims and charges a $20 payment dispute fee.
- Best strategy: most serious resellers run both, using eBay for reach and Mercari as a lower-effort second channel. Cross-list with FLUF Connect from £19/month — there is no free plan.
On this page
- Mercari vs eBay at a glance
- Feature-by-feature comparison
- Fees compared — with the maths
- The Mercari story competitors leave out
- Audience and demand
- Listing experience
- Shipping
- Returns, disputes and who pays
- Getting paid
- What real sellers say
- How to choose
- Why not both?
- Sources and verification
Mercari vs eBay at a glance
The difference is scale and intent. eBay is a 30-year-old global marketplace where buyers arrive searching for something specific, across every category, in dozens of countries. Mercari is a mobile-first US resale app where buyers browse a feed looking for deals. eBay finds you the buyer who wants your exact item; Mercari finds you the buyer who wants a bargain.
One geographic point before anything else, because it decides the question for a large share of readers: Mercari has no UK marketplace. It announced its UK exit in December 2018, stopped trading on 15 January 2019 and pulled the app that March, after a £7.3m operating loss. Mercari today operates in Japan and the US only. If you are selling from the UK, this comparison resolves to eBay.
| Mercari (US) | eBay | |
|---|---|---|
| Founded | 2013 in Tokyo; US launch September 2014 | 1995, San Jose |
| Headquarters | Palo Alto (US arm); Tokyo (parent) | San Jose, California |
| Listed | Tokyo Stock Exchange Prime: 4385 | NASDAQ: EBAY |
| Buyers | 4.3M US monthly active users (−4% YoY) | 136M active buyers (+1% YoY) |
| GMV | ~$196M in the quarter to Dec 2025 (+12% YoY) | $79.6bn FY2025 (+7%); $22.2bn in Q1 2026 (+18%) |
| Markets | Japan and the US only | Global — US $37.1bn, International $40.5bn in FY2025 |
| Seller fee | 10% flat | 13.6% typical US; £0 for UK private sellers |
| Listing fee | None, always | 250 free listings/month, then $0.35 |
| Strongest categories | Trading cards, collectibles, hobbies; mixed household goods | Collectibles and cards, motors parts, electronics, everything |
| iOS rating | 4.8★ from ~1.9M ratings | 4.8★ from ~5M ratings |
Read our full guides for depth on each: how to sell on Mercari and how to sell on eBay.
Feature-by-feature comparison
| Feature | Mercari | eBay |
|---|---|---|
| Auction listings | No | Yes |
| Fixed-price listings | Yes | Yes |
| Offers from buyers | Yes | Yes (Best Offer) |
| Offers to interested buyers | Yes — Offer to Likers, last 50 likers | Yes — offers to watchers |
| Automated price drops | Yes — Smart Pricing | No native equivalent |
| Bulk listing tools | None — single-item mobile flow | Seller Hub, File Exchange CSV, templates |
| AI-assisted listing | Yes — photo to full listing in three taps | Yes — Magical Listing, 100M+ AI-assisted listings |
| Seller tier programme | None — four badges, no fee discount | Top Rated Seller — 10% FVF discount |
| Store subscriptions | No | Yes — Starter to Enterprise |
| Promoted listings | Free tools, cost is margin not fee | Paid — pay-per-sale or CPC |
| International selling | Effectively none outbound | Yes, extensive |
| Return shipping on disputes | Mercari pays | Seller pays |
Two rows decide more than the rest. Mercari has no bulk tools at all — every listing is created individually in a mobile-first flow, which caps how fast you can scale on it. And Mercari has no seller tier programme: there is no Top Rated equivalent, no fee discount for performance, no ranking reward. You get four badges — Member Since, Fast Shipping, Reliable and Fast Responder — and that is the whole system.
Fees compared — with the maths
Start with the correction, because it invalidates a good share of what is currently published. Mercari’s zero-selling-fee experiment is dead. It launched on 27 March 2024 and was reversed on 6 January 2025. Sellers pay 10% again, and buyers pay a 3.6% buyer protection fee. Mercari’s own explanation for the reversal was that the zero-fee model “underserved buyers, wasn’t helping sellers sell more, and ultimately negatively impacted Mercari.”
Several pages ranking for this comparison today still quote either the dead zero-fee model or Mercari’s abolished 2.9% + $0.30 payment processing fee. Mercari’s own fee page is explicit: “No payment processing fee.”
Mercari US
| Fee | Who pays | Amount |
|---|---|---|
| Selling fee | Seller | 10% of item price plus buyer-paid shipping |
| Buyer protection fee | Buyer | 3.6% of item price plus buyer-paid shipping |
| Payment processing | — | $0 — abolished January 2025 |
| Listing | — | $0, always |
| Cancellation | Whoever cancels | 5% of item price, capped at $25 |
| Instant Pay | Seller | $3 flat, max $600/month, once per day |
| Standard payout (ACH) | — | Free of charge; $2 only on a failed deposit |
eBay US
| Category | Final value fee |
|---|---|
| Most categories — clothing, electronics, home & garden, motors parts | 13.6% up to $7,500 |
| Trading cards and collectibles | 13.25% |
| Athletic shoes $150 and above | 8% — and no per-order fee |
| Books, films, music (excluding vinyl) | 15.3% |
| Women’s bags and handbags | 15% up to $2,000, then 9% |
| Jewellery and watches | 15% up to $5,000, then 9% |
| Guitars and basses | 6.7% |
Add a per-order fee of $0.30 on orders up to $10 and $0.40 above, an international fee of 1.65%, and 3% currency conversion. A $20 payment dispute fee applies, waived if you refund proactively. Sellers who fall Below Standard pay a 6% surcharge, and from 1 July 2026 an additional 7% after four or more consecutive months at that level.
eBay UK is a completely different model
Since October 2024, UK private sellers pay no seller fees at all on domestic sales. eBay charges the buyer a Buyer Protection Fee instead. As restructured on 17 July 2025 that is £0.10 fixed plus 7% up to £20, 4% from £20 to £300, 2% from £300 to £4,000, and nothing above that. Private sellers still pay 3% on international sales, and the arrangement excludes Motors and sellers above 300 listings a month.
UK business sellers work differently again: buyer protection is bundled into the seller’s final value fee, so the buyer pays nothing extra. Clothing, shoes and accessories run at 11.9%, plus the per-order fee, a regulatory operating fee of roughly 0.35%, and 20% VAT on all fees — reclaimable if you are VAT-registered.
There is no eBay US equivalent of the UK buyer protection fee. Do not read US guidance as applying to the UK, or vice versa.
What you actually keep
- Mercari: fee base is $30.00 + $7.48 = $37.48. Seller pays 10% of that = $3.75. You keep $26.25. The buyer pays $38.83.
- eBay US (clothing, free shipping, no store): 13.6% = $4.08, plus $0.40 per-order. You keep $25.52.
- Difference: 73 cents. Take Instant Pay on Mercari and eBay wins.
- eBay UK private seller: £0 in seller fees. You keep £30.00. The buyer pays £31.90.
- eBay UK business seller (clothing, 11.9%): £3.57 + £0.40 + £0.11 regulatory = £4.08 ex-VAT, £4.90 with VAT. You keep £25.10, or £25.92 if you reclaim the VAT.
- Mercari: not available. No UK marketplace.
The honest conclusion is one no competing page states: on a typical US item, fees are not a reason to choose between these platforms. The real differences are demand volume, dispute handling and payout timing.
One genuine category exception is worth knowing. eBay’s 8% rate on athletic shoes above $150, with no per-order fee, is the single best rate on either platform — for sneakers it is not close. Going the other way, Mercari’s 49¢ First-Class envelope rate for trading cards, extended to 31 December 2026, is a structural cost advantage on $2–5 singles that eBay cannot match.
The Mercari story competitors leave out
Not one page ranking for this comparison covers what has actually happened to Mercari US, and it is the thing a returning seller most wants to know.
| When | What happened |
|---|---|
| March 2024 | Zero selling fees launched — the first major US marketplace to do it |
| June 2024 | Around 45% of US full-time staff laid off. The CEO’s memo cited “poor business performance” and said the fee change “helped us increase listings, but have not yet delivered the short-term results we had hoped for” |
| Q4 2024 | US GMV down 27% year on year |
| November 2024 | US CEO resigns; the founder takes direct control of the US business |
| 6 January 2025 | Fees reversed to 10% seller / 3.6% buyer |
| Year to June 2025 | First-ever full-year profit for the US business |
| Quarter to Dec 2025 | GMV up 12%, but monthly active users still down 4% |
| June 2026 | Standalone Mercari Global App launched in the US — a growth signal, not an exit signal |
The fair summary is profitable but shrinking: fewer buyers, spending more. That matters for how you use it. Mercari is not a dying platform, but it is not a growing audience either, and sellers who remember 2023 volumes will not find them.
Audience and demand
The scale gap is the single most important number in this comparison. eBay reports 136 million active buyers. Mercari reports 4.3 million US monthly active users. That is roughly thirty to one, and no fee advantage compensates for it if your item is one that needs finding.
| Mercari | eBay | |
|---|---|---|
| Buyer intent | Browse and discover — deal-seeking | Search — specific item in mind |
| Gender split | ~55% male / 45% female | ~60% male / 40% female |
| Largest age band | 25–34 | 25–34 |
| Session depth | 7.3 pages, 6m44s average | 6.7 pages, 6m03s average |
| Trajectory | GMV growing, users declining | Both growing modestly |
Mercari’s deeper sessions and lower bounce rate are consistent with the browse-versus-search framing, though neither platform publishes intent data, so treat the causation as inference rather than fact. Note also that publicly available traffic estimates for mercari.com are dominated by Japan — around two thirds of it — so headline visit figures materially overstate the US marketplace.
Neither company publishes average order value. Any comparison quoting one is repeating blog consensus.
Listing experience
| Mercari | eBay | |
|---|---|---|
| Photos | Up to 12 | Up to 24 |
| Taxonomy | Shallow, single category, AI-suggested from the photo | Deep, with required item specifics per category |
| Search behaviour | Description text is searchable — you need not cram the title | Title-weighted with item specifics |
| Bulk | None | Seller Hub, File Exchange CSV |
| AI assist | Photo to listing in three taps; about one in three first-time sellers uses it | Magical Listing — over 95% of sellers keep the AI-written description |
The searchable-description difference is genuinely useful and almost never mentioned. As one seller put it: “the body of the listing is included in search results so you don’t have to cram everything in the title; and listings are more likely to show up on google searches than ebay ones.”
The counterweight is that Mercari’s search is widely criticised as weak. Sellers report that typos, hyphens, punctuation without spaces and abbreviations simply do not match, and that Mercari does not retire old listings — one seller found a live listing from 2016 while browsing Xbox games. Sold listings also show by default.
Visibility mechanics
On eBay, Best Match weighs relevance, item popularity, price, seller performance and listing completeness. Good-Til-Cancelled auto-renewal gives no boost — no new listing ID and no position reset — whereas ending and relisting creates a new listing ID and does appear to refresh standing. Revising preserves history and refreshes nothing.
On Mercari, freshness is the whole game and the levers are built in and cost nothing directly: Promote requires at least a 5% price cut and bumps you in search while notifying likers; Offer to Likers needs 10% off the item’s historical low and goes to the last 50 likers without changing the public price; Smart Pricing drops automatically toward a floor you set; and Time-Limited Sales, added in September 2025, run 72-hour discounts with a red timer badge and enhanced search visibility on listings live for at least seven days.
The cost of all of those is margin rather than a fee — which is arguably a cleaner deal than eBay’s promoted listings, where you pay a percentage on top of the final value fee.
Shipping
| Mercari | eBay | |
|---|---|---|
| Who pays | Seller chooses per listing | Seller chooses per listing |
| Carriers | USPS, UPS, FedEx | Multiple, via eBay Labels |
| Label source | Mercari prepaid labels | eBay Labels or your own |
| Weight handling | Rounds up to the nearest pound above 1 lb — a 3.25 lb item needs a 4 lb label | Actual or calculated weight |
| Damage protection | Up to $200, requires a Mercari label and a carrier receipt within 3 days | Via Money Back Guarantee |
| International | Effectively none outbound | Yes |
Mercari’s rate card favours light parcels. A 1 lb USPS Ground Advantage label is $7.48 and a 5 lb UPS Ground Saver label is $7.97, with weight and size caps — 5 lb and a 17-inch longest side on the cheapest bands — that make it strong for clothing and cards and poor for anything bulky. The pound-rounding rule is a real trap: weigh packed, not bare.
eBay’s international reach is the structural advantage. Mercari’s only cross-border feature was inbound, letting US buyers purchase from Japanese sellers, and that ceased after June 2026 in favour of the separate Global App. As an outbound channel for a US seller, Mercari is domestic.
Returns, disputes and who pays
This is where the platforms diverge most, and where most comparison articles say nothing at all.
| Scenario | Mercari | eBay |
|---|---|---|
| Buyer window to complain | 72 hours from carrier-confirmed delivery | Money Back Guarantee timeframes |
| Buyer does nothing | Funds auto-release, sale is final | No universal auto-release timer |
| Genuine not-as-described claim | Mercari pays the return label | Seller pays return shipping |
| Buyer changed their mind | Not a valid reason — refused | Governed by your own returns policy |
| Who decides | Mercari — the seller cannot decline | eBay, with a 30-day appeal window for either party |
| Dispute fee | None | $20, waived if you refund proactively |
| Items over 50 lb | Seller must supply the return label within 3 days | Standard rules apply |
Two things here run against the received wisdom. First, Mercari’s 72-hour auto-release is genuinely seller-favourable — if the buyer does not act, the sale closes and the money is yours. Second, Mercari pays return postage where eBay makes you pay it. Combined with eBay’s $20 dispute fee and its Very High INAD surcharge, a returns-prone category costs materially more to run on eBay.
The counterweight, and it is a real one: Mercari’s seller protection documentation is thin. There is no published adjudication process for not-as-described claims, no documented seller appeal route, and no statement about what happens to funds during a dispute. eBay’s Money Back Guarantee, whatever sellers think of its outcomes, is at least written down.
Getting paid
Mercari holds your money until the buyer rates the order, or 72 hours after carrier-confirmed delivery if they never do. Instant Pay costs a flat $3, capped at $600 a month and once per day. Standard ACH is free of charge and takes up to five business days, with a $2 charge only if the deposit fails.
eBay runs scheduled payouts, daily or weekly, landing in one to three business days, with an express option at $2 flat. UK private sellers meeting a modest track record — at least ten completed sales totalling £150 over five years, with no more than two open cases — have had funds within 24 hours since August 2025.
The waiting-for-a-rating mechanic is the single most-repeated Mercari complaint among sellers, and appears in none of the top-ranking comparisons: “I hate having to wait for a rating to get paid. Waiting for delivery would be more acceptable than a rating.”
What real sellers say
On how the two platforms differ in character:
“I sell on mercari and ebay and mercari is such a different customer base. Sometimes there’s crossover but overall it feels like ebay is where people go for expensive/rare/niche/htf things and mercari is for people who want deals. You can list something worth 100 for 50 and someone will immediately send an offer for 45.”
— Reddit seller, r/Flipping, June 2026
From a seller who ran an actual three-month test across platforms:
“eBay: Highest final prices, most buyer protection headaches. One ‘item not received’ claim despite tracking showing delivery ate an entire month of profit margin. Best for rare/collectible items where price matters more than speed. […] Mercari: Lowest final prices on average, least friction by far. List, ship, done. No social performance required. […] For casual selling of mixed household items, Mercari won for me purely on time-per-dollar. For anything valuable, eBay.”
— Reddit seller, r/Mercari, February 2026
On how sellers actually split inventory between them:
“category first, then buyer behavior. ebay for reach, etsy for vintage or collectible where buyers are already priced up, facebook for bulky stuff where shipping kills the margin. mercari as a clearance channel when something hasn’t moved.”
— Reddit seller, r/Flipping, June 2026
On eBay’s downsides, in a seller’s own words:
“Literally every other item is causing issues now. Buyers assume you are a business, even with a couple of hundred feedback, and expect to be able to change their mind and send stuff back and have you absorb the postage. I’m absolutely done with it.”
— Reddit seller, r/eBaySellers, June 2026
And an important competitive note neither platform’s own marketing will tell you — Vinted and Depop are visibly taking Mercari’s lighter apparel volume:
“I moved a bunch of my stale inventory from eBay and Merc over to vinted and started selling the same day. […] I really think the free shipping is boosting more sales over there.”
— Reddit seller, r/Mercari, July 2026
How to choose
- Sell mixed household goods, mid-range clothing or trading cards in the US
- Value time-per-dollar over final sale price
- Ship light — under 5 lb is where its label pricing wins
- Want the platform to absorb return postage on approved returns
- Would rather not manage a store subscription, seller tiers or promoted-listing bids
- Sell rare, collectible, branded or high-value items where the right buyer must find you
- Sell sneakers — the 8% rate above $150 with no per-order fee is unmatched
- Are outside the US, where Mercari simply does not operate
- Need bulk listing tools, auctions, or international reach
- Are a UK private seller, where your seller fees are zero
By category, briefly: sneakers go to eBay on fees alone. Trading cards are genuinely competitive — eBay’s 13.25% and its position as the biggest GMV growth driver in Q1 2026, against Mercari’s 49¢ card envelope. Collectibles and rare items go to eBay, on buyer pool. Mixed household goods favour Mercari on effort. Bulky items suit neither — sellers route those to Facebook Marketplace.
We are not declaring a winner, because there isn’t one. The two platforms are now close enough on fees that the decision turns entirely on what you sell and how much of your time it deserves.
Why not both? Sell on Mercari and eBay together
Most experienced resellers do not choose. They list on eBay for reach and use Mercari as a second surface, often as a clearance channel for items that have gone stale. The obstacles are practical rather than strategic: duplicated listing effort, and the risk of selling the same item twice.
That second one is not hypothetical. As one seller described it: “My thing with cross listing is constantly making sales on [eBay] (not hearing/getting the notification) so I rush to Mercari hours later and delete the item because I worry it might double sell. Kind of stressful.”
That is exactly the problem inventory sync exists to solve. FLUF Connect lists once and publishes to both, and when an item sells on either, it comes down on the other automatically.
What FLUF Connect supports on each
| FLUF Connect feature | Mercari | eBay |
|---|---|---|
| Crosslisting | Yes | Yes |
| Sold here → delisted elsewhere | Yes | Yes |
| Sold elsewhere → delisted here | Yes | Yes |
| Auto-relisting | Not yet | Yes |
| Offer management | Not yet | Yes |
| In-place price edits | Yes | Yes |
| Bulk operations | Yes | Yes |
| Connection method | Browser extension required | Server-side, no extension |
Two honest caveats. Mercari is an extension-first channel — it needs the FLUF browser extension installed and signed in, where eBay runs entirely server-side. And relisting and offer management are not yet available on Mercari: Mercari’s own in-place edit surface currently covers price but not title, description or size, so a full relist cannot be automated safely. Price edits do work in place, which means a price drop preserves the listing’s age, likes and views rather than resetting them.
What it costs
| Plan | Price | Products | Includes |
|---|---|---|---|
| Growth | £19/month | 1,000 | All automation features |
| Seller | £99/month | 5,000 | All automation features |
| Super Seller | £299/month | Unlimited | Priority sync |
There is no free plan. Automation is included in every plan rather than sold as an add-on, and every plan covers all supported channels, not just these two. See pricing, our comparison against Vendoo, and the guide to selling on multiple platforms.
Sell on both — start crosslisting
Sources and verification
- Mercari — Selling fees — the 10% seller fee, 3.6% buyer protection fee, and confirmation of no payment processing fee
- Mercari — the fee change explained — Mercari’s own account of reversing zero fees
- Mercari — shipping rates and carriers and weight rules
- Mercari — rating window and returns — the 72-hour rule and who pays return postage
- Mercari — seller badges
- eBay — selling fees — US final value fees by category, including the 8% athletic-shoe rate
- eBay — 2026 selling fee changes — the additional Below Standard surcharge from 1 July 2026
- eBay Money Back Guarantee policy — return shipping liability
- eBay — Top Rated Seller — the 10% final value fee discount
- eBay Q1 2026 results — 136M active buyers, GMV
- Mercari — Global App launch, June 2026
- EcommerceBytes — Mercari’s UK exit
- IRS — 1099-K threshold restored to $20,000 and 200 transactions
- Seller quotes from public Reddit threads in r/Flipping, r/Mercari and r/eBaySellers, dated 2025–2026, quoted verbatim
Figures verified against primary sources in August 2026. Neither company publishes average order value or buyer income data, and eBay publishes no category GMV split or promoted-listing organic-lift figure — those are deliberately absent above rather than estimated. UK business final value fees are drawn from third-party summaries of eBay’s published schedule; check your own fee table before pricing.
Related guides
- How to sell on Mercari — the full Mercari guide
- How to sell on eBay — the full eBay guide
- Poshmark vs Mercari — the other US resale comparison
- Crosslist from Mercari to eBay — the field mapping in detail
- Inventory sync — how to sell on both without overselling
- Crosslisting hub — every supported channel pair
Frequently Asked Questions
They are now almost identical in the US. On a $30 item with buyer-paid shipping you keep $26.25 on Mercari (10% of the item plus shipping) and $25.52 on eBay (13.6% plus a $0.40 per-order fee) — a difference of 73 cents. Use Mercari's $3 Instant Pay and eBay comes out ahead. The fee gap that most comparison articles lead with is effectively gone.
No. Zero selling fees launched in March 2024 and were reversed on 6 January 2025. Sellers pay 10% and buyers pay a 3.6% buyer protection fee. Mercari's own explanation was that the model "underserved buyers, wasn't helping sellers sell more, and ultimately negatively impacted Mercari." Mercari also abolished its 2.9% + $0.30 payment processing fee — several pages ranking for this comparison still quote it.
No. Mercari announced its UK exit in December 2018, stopped trading on 15 January 2019 and removed the app that March, after a £7.3m operating loss. It operates in Japan and the US only. For a UK seller the comparison resolves to eBay — and note that UK private sellers on eBay pay no seller fees at all on domestic sales, with the buyer paying a protection fee instead.
eBay, by roughly thirty to one. eBay reports 136 million active buyers globally. Mercari reports 4.3 million US monthly active users, down 4% year on year. That gap is the single most important number in this comparison: no fee advantage compensates for it when your item is one that a buyer has to find rather than stumble across.
This runs against the usual assumption. On an approved not-as-described return, Mercari pays the return label — the seller does not, except on items over 50 lb. On eBay, the seller bears return shipping for item-not-as-described claims, and there is a $20 payment dispute fee (waived if you refund proactively). For returns-prone categories, eBay costs materially more to run.
Mercari holds funds until the buyer rates the order, or releases automatically 72 hours after carrier-confirmed delivery if they never do. Instant Pay costs $3 flat, capped at $600 a month. eBay runs scheduled daily or weekly payouts landing in one to three business days, with a $2 express option. The wait-for-a-rating mechanic is the most-repeated Mercari seller complaint and appears in none of the top-ranking comparisons.
It is shrinking in users but no longer losing money. The arc: zero fees in March 2024, roughly 45% of US staff laid off that June, GMV down 27% in Q4 2024, the US CEO resigning that November, fees reversed in January 2025, then a first-ever full-year profit for the year to June 2025. In the quarter to December 2025 GMV was up 12% while monthly active users were still down 4%. A standalone Global App launched in the US in June 2026 — a growth signal, not an exit one.
Sneakers go to eBay without much argument: 8% final value fee on athletic shoes above $150, with no per-order fee, is the best rate on either platform. Trading cards are genuinely competitive — eBay charges 13.25% and collectibles were its single biggest GMV growth driver in Q1 2026, while Mercari's 49¢ First-Class envelope rate, extended to 31 December 2026, is a structural cost advantage on cheap singles.
Yes, and most experienced resellers do — typically eBay for reach and Mercari as a lower-effort second channel or clearance surface. The practical risk is selling the same item twice, which sellers describe as a genuine source of stress. FLUF Connect lists once, publishes to both, and delists automatically on one when the item sells on the other. Plans start at £19/month (Growth — 1,000 products); there is no free plan.
Not yet on Mercari — both are available on eBay. Mercari's own in-place edit surface currently covers price but not title, description or size, so a full relist cannot be automated safely and we would rather not ship something that mangles listings. Price edits do work in place, which means a price drop keeps the listing's age, likes and views instead of resetting them. Note also that Mercari is extension-first: it needs the FLUF browser extension, where eBay runs entirely server-side.