Since the OP is a casual seller selling a high dollar quick flip scammable item, it is pretty obvious that the buyer is attempting to scam.
OP, once the buyer found out that you are agreeable to negotiation, they just paid and are now continuing to negotiate.
What you do is to not justify anything, the buyer knows you didn't change anything and just give her one option - return for refund. Tell them to file a case with ebay so you can send them a return shipping label.
If they were telling the truth, they would have file a not described case against you already. Which tells me that they don't want to get on ebay's radar any more than they have to OR they have been kicked out of buyer protection. What they are doing is hoping that you will be so flustered that you ill just send them some refund money to make them o away.
If they are new at scamming, they might just give up and move on. But if they are experienced, you might not get back your boots when the buyer returns something. This is the downside to selling on ebay - the scamcentric policies. If the buyer files a dispute and returns their beat up old tennis shoes, you have to refund as if you got YOUR boots back - that is the ebay way. At that point, you have to file police and mail fraud reports and perhaps take the buyer to court.
There is a reason my signature line is what it is.
(*Bleep*)
