Please stop with the whole "prove the handgun was sold" thing as some kind of defense against a hypothetical criminal act committed with a firearm once associated with yourself.
There's FAR more to a criminal investigation, criminal charges, and prosecution than "this dude once owned this firearm". At the very basic level, there's means, motivation, and opportunity involved here, not to mention whatever forensic evidence in play, witnesses, and far, far more.
Even the fact that any given firearm was actually transferred to someone else via FFL and Form 4473 does not mean that person RETAINED that firearm, that he did not even sell or gift that firearm to any other given person INCLUDING back to the original owner.
If the burden of proof were that low, then simple hearsay would carry the day anytime someone investigated a criminal act.
That said...if anybody WANTS to add additional criteria to a sale of a firearm, so be it. It's their private property and so long as they conduct the sale in accordance with their jurisdictional laws, they are free to set any criteria they wish for any reason. The buyer will decide for himself if those terms are acceptable or not.
Just quit fooling yourself that that all these extra measures are some kind of magical ward against being the subject of an investigation or potential prosecution.