I am pretty sure the buyer got the real headphones and sent back some fakes.It's probably a little too late for the card company to do a charge back but they were fake and if you represented them as legit don't you think it would be the right thing to give the guy his money back?
As to eBay and what they would do I have no clue... but to keep everyone happy and not lose your eBay account I would probably just refund him. After all you do say you made a decent buck at that and this is part of the cost of doing business.
I was curious if I could send him a polite message, explaining I am unable to refund
I might offer him half, at least. Bear in mind he had 6 months of use from an item he obviously enjoyed using. This also saves him going through eBay/bank. That's kind of fair, in a corrupt, warped kind of way..I'll think about what I'm going to do...