You can easily end up with A gifting B a million and then B sending A the NFT for free, potentially with a trusted escrow service in between to make sure both of these actually happen. The NFT marketplaces are essentially already acting as escrow, so this isn’t weird.
Only thing you could probably enforce is that moving something from one key to another requires a fee to be paid to the original artist, but that’d also trigger if A wants to move their assets to a different key (eg in or out of some hardware wallet, online wallet or marketplace). And if A and B trust each other strongly they can simply share the key.
You can easily end up with A gifting B a million and then B sending A the NFT for free, potentially with a trusted escrow service in between to make sure both of these actually happen. The NFT marketplaces are essentially already acting as escrow, so this isn’t weird.
Only thing you could probably enforce is that moving something from one key to another requires a fee to be paid to the original artist, but that’d also trigger if A wants to move their assets to a different key (eg in or out of some hardware wallet, online wallet or marketplace). And if A and B trust each other strongly they can simply share the key.