In other words, aside from simple goodwill, what enforcement mechanism would there be to make sure that John Doe A transferring to John Doe B goes through an FFL?
The same we we make sure Mom-And-Pop Rifle company sells to Joe's Guns, who sells to John Doe. When/IF they find the firearm and have to trace it, they follow the record trail. When John Doe A can't produce the firearm, nor point to an FFL dealer with the 4473 record, he'll be like Lucy when Ricky gets home.. having a lot of 'splainin' to do.