I think that something like this could actually be a useful part of a deal to get “universal background checks”. Not necessarily a “smartphone app” as such, but some kind of federal government web site (along with a toll-free phone number for people who are still marooned in the 1990s) whereby if Alice wants to sell Bob one of her guns (Sure-Fire Firearms Model X, serial number XYZ123), she can enter Bob’s info and get back a “YES, you may lawfully convey a firearm to this person” or a “NO, you may NOT lawfully convey a firearm to this person” message. Alice would then get some kind of receipt with all the relevant info about the transaction–along with a fancy barcode on it to prove authenticity–that she can keep to prove she did her due diligence. I suppose Bob could get a print-out too, if he wants one, but it wouldn’t be as relevant for him.
So, Sure-Fire Firearms Model X, serial number XYZ123 gets used in a crime. (Maybe the cops find it right at the scene and everything.) The ATF goes to Sure-Fire and says “Who’d you sell #XYZ123” to? They reply “Oh, that was part of a shipment to Joe’s Gunshop in Anytown, USA, FFL #4567890”. ATF goes to Joe’s Gunshop and says “Who’d you sell Sure-Fire Firearms #XYZ123 to?” Joe pulls his Form 4473 and gives them Alice’s info. (Up to this point, this is how I understand the system works right now.) So, ATF goes to Alice and says “Hey, do you still have Sure-Fire Firearms #XYZ123?” She says “Why, no, I sold it to Bob. Here’s my receipt from universalbackgroundchecks dot gov”. So, on to Bob. Maybe Bob has a receipt of his own, and the ATF goes to track down Charlie. Maybe Bob says “Oh, that’s my gun that was stolen–here’s the police report”. Or maybe Bob doesn’t have a receipt from universalbackgroundchecks dot gov, or any other lawful account of where Sure-Fire Firearms #XYZ123 wound up (“I transferred it to my son, Bob, Jr., in accordance with the Close Family Members Exception of the Universal Background Checks Act”), in which case Bob is in Big Trouble (he’s actually guilty of the crime that’s being investigated) or at least in Serious Trouble (he has an iron-clad alibi for the crime that’s being investigated, but is still looking at federal charges for a gun sale that he didn’t bother to do a background check on, where the buyer wound up committing a crime with that gun–even if Bob gets no prison time, since we’re trying to reduce “mass incarceration” in this country, he’s still saying “bye-bye” to any right to legally own guns himself).
(Note that Bob could have passed the background check, but still be the criminal. “Past Performance Is No Guarantee of Future Results.” But on the other hand, after they give you the PPINGOFR disclaimer, all the stock prospectuses give you information on past performance, because what else can they do? So, Bob is arrested, but Alice and Joe’s Gunshop and Sure-Fire Firearms are all off the hook, since they all did what they were supposed to do.)
So, universalbackgroundchecks dot gov has a big screen that says “You agree, under penalty of perjury, that you are conducting this background check as part of a lawful firearms transaction to which both parties agree” (or something like that) and you have to check a little box which counts as your e-signature. Of course Mrs. Grundy might just ignore that statement, and I doubt much effort would be put into actually tracking down people who abuse the system. But realistically, what does Mrs. Grundy get? She gets the biographical information on Dave, next door. (Maybe she goes through his garbage on trash day.) She commits some kind of federal crime by running an unauthorized background check on him. And…she gets a message that says “YES, you may lawfully convey a firearm to this person”–which doesn’t actually tell her if Dave has any guns or not, or anything much about Dave’s medical and mental health history beyond that he hasn’t actually been involuntarily committed, and doesn’t tell Mrs. Grundy if Dave was ever tried for a felony but acquitted, or if he was charged with a felony but the charges were dropped. Or, Mrs. Grundy runs Edna (from the house on the other side of her) through the system and gets the “NO, you may NOT lawfully convey a firearm to this person” message. So, Mrs. Grundy has committed a federal crime, and found out that Edna is a serial killer. Or that Edna was convicted of lying to the feds about an insider trading case ten years ago. Or that Edna spent a few weeks in a mental hospital when she was in college. Or that Edna was charged with a felony, but the charges were dropped, but the Podunk P.O. are lousy at record-keeping and reporting and poor Edna may be in for a Big Surprise someday when she gets pulled over for a routine traffic stop. Or that Mrs. Grundy screwed up Edna’s biographical info and got a background check for some other person. Note that there’s no reason why the system would need to be set up to give a user (authorized or not) more than a “YES/NO” decision, so Mrs. Grundy (who, again, has knowingly violated federal law) hasn’t actually managed to get that much information for her (potentially serious) trouble.