You said PPT so I assume it's private party transfer. There should be no shipping back to original owner since it's a face to face meet and the gun must be DROSed again.
If the buyer fails the DROS, the gun will now have to be re-DROS again to the original owner. If the original owner fails the DROS, the gun will be forfeited to the store. The store will have a couple of options, sell it or turn it in to PD to be destroyed, etc.