The seller should have anticipated this and either said "no refund, no inspection period" or he should have said "if it comes back for any reason the buyer eats the fees to return it to the seller". Figure that the seller already has his money and isn't too inclined to do much at this point. If the "buyer and seller" have agreed that it will be returned, then they should have talked about how to accomplish this.
Also, I wonder what obligation the receiving FFL has to log the gun into his bound book? He should, since he accepted it on behalf of the buyer, but what happens when the receiving dealer gets a gun in the mail and says "I didn't order this"? Interesting question.
In any event, it's really up to the buyer and seller to resolve this issue. If neither party want to help, the buyer is gonna be hosed. Figure the seller already has his money. If I'm the receiving dealer, I'd tell the buyer, "you gotta make this seller provide FFL data and pay for postage to get it back to him. if you don't do that within x days, I'm charging YOU $x per day for storage". At a certain point he'll have the value of the gun in storage fees.
All together, though, a bad situation that could have been handled by a written agreement between buyer/seller or buyer/local FFL. Bet the local FFL won't be receiving guns for pals any time soon.
As an aside, who expects an inspection period on 99% of the Title I guns out there? I've bought expensive guns sight unseen and have lived with a few lumps and scratches along the way. Imho, if you're not prepared to lose every dollar when buying anything on the internet, you shouldn't be buying on the internet. For high dollar items (say, $10,000 or more - although everybody will have a different threshold here...), it's not unreasonable to spend $300 or so and jump on an airplane and go see the item in question before forking over major dollars. We're probably talking about a gun worth less than a grand here. Probably a lot less than a grand. Figure the buyer is the guy going to come out on the losing end here - consider it a stupid tax.