What are my options when a case is opened against me on ebay?

[bstuek]

So, almost a month ago, (Feb. 11th) I sold an autographed baseball to a man. For some reason, he just now opened a case, claiming that "the signature of Albert Pujols on the baseball does not match his known signature. The letter A he sighed looks nothing like the A he normally signs are the certificate of authencity is not a offical form. Would like a refund"...... word for word...Problem is, I included very clear pictures of both his autograph AND the COA, and he STILL bought it. I sent him a message back, but he'll probably continue to be an idiot. I DON'T want to refund him anything, but I don't want negative feedback as it would GREATLY affect my feeback (currently have 11 positive). Is there any way I can keep the money and NOT get negative feedback? Thanks.
Here's a link to the item. http://www.ebay.com/itm/221371999863?ssP...1559.l2649
AND LOOK AT THIS!!!!!!!!! he's trying to sell it while he has a case opened!!!! http://www.ebay.com/itm/121288658567
ALSO- if I win the case, can he then leave negative feeback for me? He hasn't left any yet.