If it had been shipped to your FFL to be repaired, then the FFL could return it directly to the owner. Since the owner sold it, legally it is not his anymore and if returned it would be returned to the FFL closest to the seller to have it transferred back to him.
That said, is it abused so much that it can't be repaired? If not, is the seller ok with giving you part of your money back to have the repairs done? If it can't be repaired, the seller should refund you your money and your FFL needs to ship it back to an FFL of the sellers choice.