This should just be a more of what you should expect when selling parts, not something that comes down to one individual.
Yes, I have had this same guy back out of a deal I had with him. We talked down on the price to a very good deal, because I wanted the part gone, then he backed out after saying he could paypal. If I had never sent a PM saying that I had another potential buyer and needed to know the status asap, I think I'd still be waiting for a payment.
However, you would not want to know how many people I have dealt with that said they would pay, but never do. I have even went so far as to having the parts boxed and ready to be shipped, but the payment never came.