I've been wrestling with this one all day, as a California resident, to see if I can legally buy a long gun out of state from an FFL and bring it home. I've come to agree with the CA gun expert consensus that this isn't legal, but for reasons of limiting CA law, not Federal law per se.
It seems clear the seller violates 922(a)(5), in the OPs original scenario, but I don't see that the buyer has violated any laws. 922(a)(3) forbids bringing a gun back that has been purchased out of state (unless in a face to face transaction with an FFL). 922(a)(1) forbids making a business out of guns or ammo without being licensed. Those don't seem to apply in the given hypothetical.