Sometimes unscrupulous sellers will do this to try and get in a bidding war or try to squeeze more cash from you. Walk away, this guy can't be trusted.
He may try and come back and say the second buyer backed out and try and reinstate your deal. Walk away.
Research who you are buying from. Do a name search on Pinside and read his posts to get idea of who your are dealing with. COD purchase has it's pitfalls. Look into Escrow.com It protects both buyer and seller.