To sell an NFT on OpenSea, open up your OpenSea profile and then select the NFT you would like to sell. Press the Sell button, then select the option Fixed Price and fill out the price and duration you would like to sell for. Alternatively you could put your NFT up for auction and allow people to bid for it. The first time you sell an NFT, there will be two one time fees you will be required to pay, so keep this in mind when deciding to sell your NFTs.