If you bought an NFT/item from someone and want to resell it
If you minted your items but didn't list them for sale
If you have an NFT in your wallet from another service/game/app and want to sell it
If the item isn't in your wallet, then you cannot sell it. So make sure you own/have the blockchain item inside your wallet when trying to sell it. If you minted it on Mintable and haven't listed it for sale, or bought a new item, it will be in your wallet.
If you don't see your item in your wallet - you can add it directly by its smart contract address.
Click on "Sell" at the top of the page
Once loaded - you can fill out the listing information.
See "How to create an item to sell" for details