When selling parts how long do you wait for a money order from a person you agree to a deal on before you just sell the parts to someone else?

It's now been three weeks and now I can't get the buyer to respond to me.
He was promised to mail a money order on February 19th, that didn't happen and then said it would go out on the 28th.
Still no money order and he hasn't responded to any messages.

Someone else wants the parts and will PayPal the money right now.

I'm trying to be fair to the first guy, but it's getting old waiting to see if anything will actually show up.