>What if I sell my car and don't transfer my NFT showing ownership of the car? Who will enforce the requirement that I must transfer the NFT when I sell my car

Buying a car without getting the NFT would be as dumb as buying a car and not getting the title. Sure it's possible to do but it's a pretty basic fundamental step and if you don't trust yourself to do it then maybe just go to a dealer and let them take care of you.