I think the argument is that the buyer wouldn't know whether you still have a copy of the key or not. The idea is that the blockchain is used to ensure you cann

I think the argument is that the buyer wouldn't know whether you still have a copy of the key or not. The idea is that the blockchain is used to ensure you cannot backdate a transaction. Mind you, I'm not a blockchain fan, and I believe that blockchains and cryptocurrencies are the absolute worst way to solve the problem.