Depends how much hassle the op want's tbh, at the mo he has it back and can resell it if he gets in to a long winded battle with this guy it could be weeks without a resolution
This is where my inner evil comes out.
You'd have nothing to lose. You have the PSU and the money and the buyer needs to pay you to send it back or will try to argue it out.
I'd drag it out to teach them a lesson.