Why can't you sell it for some crypto asset and exchange that for cash (obviously this second exchange requires a trusted third party)? Maybe I don't understand your example.
That seems like an issue with cash (no way to enforce you giving me the thing I paid for, and no way for you to enforce me giving you the cash for the item you gave me).
Yes using cash gets rid of the guarantee that the transfer of goods is fully atomic that crypto generally offers. Buying tomatoes at the store has the exact same problem.
Let me explain: you can have a smart contract where you get 5-20% of the value whenevder it's sold and that'll work and be guaranteed (assuming the network isn't compromised eg 51% attack). That is wholly continaed with the blockchain.
But what if someone wants to sell that for cash? Now you've introduced the exact same trust issues that exist in every transaction in the traditional finance system: trust in the institutions involved and the potential needs for courts to enforce contracts.
So what exactly have you gained? Nothing. Literally nothing.