I've had a few piss-takers on ebay and it's infuriating. I have had one guy take the case up with ebay but through constant reiteration of the point 'it was sold to the best of my knowledge and until the buyer proves otherwsie it's not what it was sold as' then he cannot go any further.
You sold this to the best of your knowledge and surely he has to prove 100% otherwise & that he is right.
Regarding tsx v slx foz above is on the mark. Internal rifling is likely to be central, where the tubing is thinest not all the way to the ends like slx. Has he stuck a torch down the seat tube and taken a look? it's easy to see all the way down that way, get him to take a camera pic down there and email it to you.
Ask him to supply you with a link that states tsx tubing has rifling all the way to the ends.
**Withdraw the money from paypal**. It may be a pain to end up with negative feedback but you soon build it back up again, you may of course not be too worried about that and if you have little feedback then just start a new ebay account.
I kept all my money and despite negative feedback it hasn't harmed my selling. I've had people saying they haven't received their bits and when I reply with I have the proof of posting shall I scan it to you all goes quiet..... consider every ebayer guilty until proven otherwise
This guy bought to sell and make some money so don't feel too bad for him, be firm but fair