"This puts an arguable burden on the seller that a prosecutor can take advantage of."
Eh, it's a pretty high bar, that pretty much isn't ever met unless the two parties know each other and their respective legal disability (in which case it's usually more a straw purchase thing going on). The kind of suspicions required for a layman to disqualify someone would generally fall under unacceptable 'profiling' that in any number of plausible/common scenarios are not justifiable disqualifications in the first place (guy with a doo-rag and what appear to be gang-tats paying for his Hi Point in singles can not *reasonably* be assumed to be a prohibited person by those facts alone, and a casual private seller is unlikely to interact with him long enough to glean much more than that). Criminals don't usually announce their intent to strangers when buying, and felons buying on the down low are liable to keep mum on their disability.
So yeah, short of a dedicated BGC or extended interrogation (basically what the 4473 is if you think about it; straw purchasers screw it up frequently and queer the deal) a private seller is unlikely to discover the facts needed to reasonably conclude the buyer is disqualified. At that point I guess, they're free to break off the deal and reap whatever altercation results (a scenario all too plausible when you're talking potential ex-cons & gangbangers with poor impulse control). I don't know about you, but I personally feel that this is a rather undue burden on the seller, both due to the potential consequences, the typical lack of evidence in the interaction to make such a judgement, and because at the end of the day, whatever the buyer does good or bad with the gun, whatever his legal situation, is quite frankly not the responsibility of the seller (the seller is responsible for *selling* the gun to the ex-con, not killing the thug's girlfriend that night). I see even less how it's any responsibility of some random FFL dealer who is neither providing the gun nor facilitating the sale nor vouching for anyone involved.
I guess you can argue the FFL is in the best position to act as gatekeeper since they do this for their own products they sell, but again --we're not talking about products they sell, but a completely independent transaction between individuals that has no bearing at all on the FFL apart from some government-mandate. And yet the FFL will take on significant liability for both allowing a sale (false negative on the BCG or a future crime they can't possibly know about) and blocking a sale (retribution or accusations of prejudice)
At the end of the day, the burden is on the buyer to make sure what he is buying is legal; we're talking law-abiding citizens here, remember, since criminals can easily circumvent the entire process by choosing to forgo the BCG. The crook is the one who ends up with felon in possession charges (ironically dropped most of the time), the seller is practically always off scott-free unless it's a mighty overwhelming case of collusion. The only way to put the burden back on the seller, as is desired because paradoxically it is assumed they obey the law & thus will be a more effective vector for gun control rules, is to require the BGC. The only way to prove the BCG did not occur is to put the gun in the owner's hand at the time of illegal sale/transfer/theft. The only way to do that is through registration.
All so we can put the legal liability on *law abiding* gun owners seeking to *obey the law* so they won't *accidentally* transfer a gun to a known criminal released onto the streets who will *actually* commit the violent crime that is the supposed target of all of this. Known criminals whose 'gun crimes' these schemes seek to prevent are routinely dropped so as to reduce their prison terms as a bargaining measure. The mythical "iron river" sellers providing legal guns directly to criminals knowingly will choose to ignore the BGC and take their chances as they always have, thus mooting the entire BGC scheme that was supposedly enacted to target them.
End result, .gov has a list of who has what guns so they can prosecute otherwise law-abiding gun owners who accidentally violate this law (criminal enterprise won't bother registering the guns prior to sale, obviously), but they probably won't prosecute hardly anyone, because they hardly ever prosecute the felon in possession at the end of the day. Summary; felon still gets guns through illegal channels as they typically do already, all gun transactions now carry new legal liability for those trying to follow the law, government has a registry of firearms & owners that they've always wanted. Say, the only thing that's changed from the status quo is that registry & its enforcement!
TCB