I sold a rifle on GB awhile back and the guy misread the ad and decided he didn't want the rifle. He sent it back to me, and of course it has to come back through my FFL. My memory isn't what it used to be and I can't tell you if I had to sign the papers as if I was buying a new firearm. I do know my FFL didn't charge me a fee for the transfer.
I am not an FFL holder and I sent the rifle myself through the USPS. I sent the rifle to the buyers FFL holder. I'm no expert, but I am pretty sure the firearm needs to come back through an FFL holder.