This. The deal was done. As soon as he tried to haggle you should've reminded him that you agreed on a price and if he doesn't like that then he doesn't need to buy it. You shouldn't have re-opened the bidding.He won an auction, not the right to haggle. You got done son.
How much would he have lost if he told the buyer to FO and have to re-list?
I'd suggest filing a non-payment dispute with the aim of cancelling the transaction on the eBay side of things at least.
Don't think there's anything that can be done now - I assume this was paid in cash?