The buyer made the mistake of thinking he was dealing with StubHub rather than the seller. If he had been smart, he would have asked asked StubHub for the seller's details with the comment that he would be filing suit against either StubHub or the seller.

StubHub is a marketplace and sellers frequently sell tickets they don't own. This works out because tickets frequently drop in price shortly before the game and, for the times that they don't, sellers usually eat the loss. But this seller decided not to deliver and, for that, they should be sued. The equivalent in the stock market would be to short a stock and then not pay up when the stock quadrupled. Given that the value of the loss suffered by the buyer is roughly $5k ($6k tickets less ~$k credit at StubHub), it's a good candidate for small claims court.