NFT marketplace
An NFT marketplace is where people list, browse, bid on, and trade NFTs — OpenSea, Blur, Magic Eden and others. Underneath the gallery-like website is a settlement protocol that has to solve a tricky problem: how do you let a buyer and seller swap money for a unique token atomically, so neither can be cheated, without forcing the seller to lock the NFT away while it waits to sell?
Most modern marketplaces use an off-chain order book with on-chain settlement. To list, a seller signs an order (price, token, expiry) off-chain — costing no gas — and grants the marketplace's exchange contract approval to transfer their NFT via setApprovalForAll. The NFT stays in the seller's wallet, listed but not escrowed. When a buyer accepts, they submit a transaction carrying the seller's signed order; the exchange contract verifies the signature, then in one atomic step pulls the NFT from the seller and the payment from the buyer and swaps them. OpenSea's Seaport and the older Wyvern protocol work this way; collection-wide bids let a buyer sign an offer any matching NFT can fill.
Different models trade off differently. Approval-based listing is gas-light and keeps custody with the user, but it means a live approval to the exchange contract that, if that contract is buggy, is a risk surface. True escrow (transferring the NFT into the contract) removes that lingering approval but costs gas up front and locks the asset. Marketplaces also differ on fees, on whether they honor creator royalties, and on auction styles — fixed price, timed English auctions, and declining Dutch auctions for mints.
Listing an NFT usually does not move it; it signs an order and approves an operator. That is why a malicious 'marketplace' approval, or a forged listing signature, can be as dangerous as handing over the NFT itself.