From my little experience with buyers like that, you need flow with the buyer.
If he wanna to start a case on eBay - go with, and tell eBay that you sold used jeans as the pics attached. You don't need any proof for authentically - it's used!
If the buyer don't give up and ebay be with him - you will need to refund him.
Be honest with your buyer and I hope that he will believe you.
If he said that your thavar is not like his thavar - tell them to send a...