There is no single best marketplace. There's only the best marketplace for this item, with this goal, this week. Here's the honest shape of the landscape.
Facebook Marketplace
Free to list, local, enormous reach. Best for furniture and anything awkward to ship. You'll get lowballers and no-shows, but you keep the whole price and you can be done in a day.
OfferUp
Local-first like Facebook, with a shipping option layered on. Free when you sell locally; a fee applies when you ship. Good for mid-value household items where you want a bit more structure than a Facebook message thread.
eBay
The deepest buyer pool for anything collectible, branded, technical or unusual. Fees are real, and so is the reach — if an item has a genuine niche audience, eBay finds it. It also has the best sold-comps history, which is why valuations lean on it.
Mercari
Roughly a ten percent seller fee, ship-everything model, strong for small-to-medium goods that fit in a box. Simple to list, less haggling culture than the local apps.
Poshmark and Vinted
Fashion, and essentially only fashion — clothing, shoes, handbags, accessories. Listing a coffee table here wastes your afternoon. Within fashion, though, the audience is motivated and browsing specifically to buy.
Speed versus price
Almost every decision comes down to this trade. Local marketplaces are fast and fee-free but price-capped by whoever happens to live nearby. Shipping marketplaces reach the person who actually wants your specific thing, at the cost of fees, packing and time.
That's why GoSell asks for your goal before it recommends anything. "Sell it fast" and "Make the most money" should not produce the same list — and in the app, they don't.