The "could" was implied, I know it doesn't happen now, doesn't mean it couldn't.
Check wallet for nft proving a person owns the game (since wallet content is public), if the person sells the nft then they can't play anymore, the new owner can.
Ask people to send a certain amount of crypto from their wallet to prove it's theirs and associate it with their in game account (also becomes a way to get a cut from sales).
It's surprising it's not something that already exists since it solves the DRM issue (from the game distributor perspective).
As far as I know there are no major platforms to that allows players to sell their digital games, only in game items.