The reality for sellers is that eBay never sees the items concerned and by default accepts the buyers word for faults. If you resist, the case will almost certainly be found in the buyer's favour. You would then be penalised with an account defect as well as having to refund the buyer in full. If you didn't send the buyer a prepaid return label eBay may not even require the item to be returned.
So in short, there's not much that sellers can do except agree to the return and send the buyer a prepaid return label. You will just have to hope that what you get back is what you sent. If it isn't, ask here again for advice.