Holaplex allows you to sell existing items. This means that you will sell the NFT itself-- or in other words, the master edition. This transfers ownership of the original NFT to someone else, though the record of who created the NFT is still embedded into the blockchain, and the artist can still make money off of future sales of that NFT through commission percentages. This option is best for re-selling existing works or otherwise selling the original NFT itself.
To sell an existing item, follow the How Do I Sell an NFT on Holaplex? guide and select Sell an Existing Item as your sale type.