OP .. sorry to hear about your situation; I am going through the same but in my case, I'm the seller and the buyer filed a SNAD on me.
It's no fun for either party; I'm pretty clear in my auctions about wear & provide plenty of pictures .. in my case, I think it's just that the buyer is uneducated about the particular bag (LV Denim GM Baggy). First, she noted that is was WORN, not NEW and then indicated that it was the wrong style. Well, she's wrong on both accounts .. and even though I never used it, I did note that there was a little water spot on the strap .. would I do that if I was trying to scam anyone?
In my case, I hope Paypal DOES require that she provide 'PROOF' because they have already nicked my Paypal account for the refund that she's requested! I'm pretty peeved about that because I believe that they should wait for the dispute to be resolved before ANYONE's account is affected. I provided them with all the pictures, the auction text, and comparison pics of the 2 different styles (they will clearly see that she is incorrect).
Bottom line - caveat emptor in all online transactions! I always tell my friends, make sure to do very thorough research before buying anything and if in doubt, always ask for additional pictures ... especially when something is up there in $$$!!
Again, sorry to hear about your situation, but as I said .. it really sucks when a seller gets a SNAD filed against them when the buyer is not truthful.