Which is why I'm driving 2 hours plus to deliver a rifle for a cash payment tomorrow.Buy somebody else's gun, face to face only and cash only (there is a reason I insist on face to face cash transactions when I'm selling a gun of mine, or buying from someone else.......you haven't even seen this rifle in person yet and photos can be deceptive) UNLESS there is some way to hold the funds in an escrow account pending the mutual satisfaction of both parties. Someone who will only accept a USPS money order has an agenda of some kind. Even if it is not a nefarious agenda, I have my own agenda and I'm not going to dance to someone else's unless it is compatible with mine.
to the OP.. Guy is a flake, he had the payment, your part of the transaction was complete, his loosing the payment is his issue.. even if you decide to assist him.
No idea on the legalities of it all, but at this point were I you, I would feel the loose as having been stolen from.