The difference with NFT is that it's used for art, and art comes from a specific artist, and usually in the world of art what gives value to the art is the artist. So, if I have a thing signed from the artist, it's as good as it can get. If they want to double spend or do other things, then they will lose credibility as artists and will no longer be able to sell what they sold so there's no incentive in cheating, hence no need for a central authority or blockchain. All you need is them publishing their public ledger of what was sold to whom, no blockchain necessary.