Apart from the eBay side of things, this guy sounds like a right tool.

The transaction (contract) was between you (the OP, not cats LOL) and the buyer - item received, I assume feedback left? over and done.

What the buyer ddecided to do after that, sell it, dump it, is completely up to him as it is now legally his product.

He chose to re-sell and now the contract is between him and his buyer.

This is not a warranty thing, he can not claim from you as he may from a manufacturer as the thrid party, leave it alone, it will hopefully just go away.