report him to the Auction for fraud.. if thats what he is doing then that IS fraud.. if I offer to sell you something for $10.00 and you accept and send the money, then I change my mind.. that is fraud in most juristictions.
this is one of the reasons I NEVER buy stuff from online auctions.. there to easy to screw the buyer. the auction house doesn't care.
I would report this data to Gunbroker.com (I assume that is who you went through) see if he still has a A+ rating after that
"Question that Regulation, you might be suprised to find out it's Illegal"