The NFT Marketplace is a platform that allows digital artists, authors and creators to produce and sell special crypto-assets that represent the ownership of their digital works. These digital assets are NFT (Non fungible token).
As usual, in order to sign in you need to connect to the site through your crypto wallet. It is important that the marketplace supports the same network as your wallet. For example, to register at GrandBazar you have to create a crypto wallet in Everscale blockchain. If you want to register at Opensea you need to create a crypto wallet in Ethereum blockchain.
Different marketplaces have different functionality and different options. For example, there are marketplaces with the option that allows to buy NFT tokens for fiat, some of the marketplaces are created exclusively for digital art.
What almost all marketplaces have in common is that they take a commission for transactions carried out on the site: for the purchase, sale or transfer of NFTs.