For situations like this you can call eBay, ask to speak with merchant support. Let them know the item you took back for return arrived to you damaged or otherwise in a different condition than you sent it due to the customers negligence and they should (not always) reverse the outcome of the case in your favor. They essentially see it as yes you were wrong due to the mistake but the buyer still needs to get that item back to you in the same condition it arrived to them. Helpful factors would be if you had the buyer send a picture of what they received before returning it or if you have actual pictures in your listing (not stock pictures). When you contact eBay you will need to send pictures and a signed affidavit (they will send to you) and it should be taken care of for you. The real question is if the value of the item is worth the investment of your time on this one. A lot of sellers would chalk it up to a loss rather then sending a return label and just refund the buyer knowing they will be losing out more than they made by selling the item once you take into account original shipping, item price, return shipping. Regarding the buyer never talking to you they would have had to of opened a return case to send it back, that is your chance to speak with them, offer a partial refund, etc. If they refuse partial refund and you want the item back there is no way out of refunding everything and providing return shipping unless of course it arrives to you damaged.

I cannot stress enough, to not have situations like this happen to you, make sure you are up to date on all eBay policies. Regarding Amazon, they are worse in the sense they offer less seller support as they expect you know all Amazon policies, went through the Amazon university info and are on your game. eBay will at least help you through situations in most cases unless you are being unrealistic.