I do not trust much in the security of a film to guarantee security. It is a field for skilled people. If I was a TV conjurer I’m sure I would know a thousand ways to make fraud with that.
I made an effort to build an algorithm to send money by mail in a totally secure way.
It is not perfect, but it is bad to cheat: any attempt would result in a serious loss on both sides, no one can earn money cheating. It also introduces, but in a subsidiary way, a system similar to the one you propose (in case the seller says he has not received the payment)
Perhaps both systems can be combined.