This page explains how to list an NFT for sale on the Marketplace.
To list an NFT for sale on the Marketplace you'll have to head to your Collection. Once in your collection, toggle to 'Not for sale' NFT's , find the NFT you want to list for sale and hit the pink 'SET FOR SALE' button.
Then enter your desired currency and price and hit 'Confirm'.
Your NFT is now listed for sale on the Marketplace, collectors will be able to purchase your NFT, as well as make offers to purchase your NFT. To update or cancel your listing you will now be able to find your NFT in your collection as well, with the 'For sale' filter on.