I am told the USPS does take the idea of mail fraud seriously. First do what Striker said, then have the USPS track the m.o. to see if it was cashed. GB has contact information for members here, I think. I do not know if he is willing to get involved. He has plenty on his plate already. If it is a ripoff, we could have the convention in the guy's living room while we exact revenge.
As an aside, in over 150 transactions on auction sites (mostly ebay), I have been ripped exactly once--for a lousy $11.20. Ebay gave me the guy's home phone # and I called. The guy's son happened to be visiting, and he was a fellow cheesehead. When I explained the situation, and what I was prepared to do, over the principal of the thing, he sent me a m.o. that arrived two days later. That was one apple that fell far from the tree. So, in the end, I have never lost a nickel. $600? I'd carve his heart out if it does turn out to be a ripoff.