Item returned "claimed Defective"....

[onecarattoo]

I sold an item that before I listed it was verified 100% operational by activity that removed it from service.  I sold the item and the buyer claims it is defective and wants refund.  I have reason to believe that item being reurned is not the one I sold or has had parts in it exchanged for the good parts from my unit and now he wants refund.. Will verify when received but buyer also wants me to pay for return shipping when I sold it as free shipping.  No returns accepted were in listing.. I have no problem with a refund but to pay for return shipping for something that was operational and could have been altered before return is no good.. What is a seller to do?? Lose money??  This happened to me years ago with an outboard motor that was returned to me (Ebay) with parts missing from  it.. With over 650 positive feedbacks (no negatives) 100%, this is disheartening.. and questions my future on eBay