Similar to selling your car privately. There are some forms involved to recognize that you no longer own nor are responsible for the gun in question. It's probably a little more strict and polished now (maybe not), but it wasn't that long ago that you kept a copy in case the cops came knocking looking for the gun and a copy got filed away in a drawer somewhere for basically the same reason. I can't remember if gun stores were in charge of the records for private sales (which wouldn't make sense) or if they were filed with the town/state, but it was all physical paper in a drawer somewhere regardless. There wasn't like a system actively tracking ownership - so long as both parties had a LTC, they were okay and third party sales could be done anywhere.

True, but it's the one that I know and up until around the early to mid 2000s, you could buy a shotgun in Wal Mart. They had a whole section dedicated to firearms.

Plus, the whole selling an AR out of the trunk of a car in the Wal Mart parking lot is something that a kid I went to school with actually did in Mass. There's still plenty of regulation involved (and increasing by the sounds of it based on what you said), but at the time it basically boiled down to signing the paperwork signifying the change in ownership and resale of the firearm. The only time the state would've been made aware was if they requested to see the paperwork, AFAIK.

Besides, the vast majority of people 3d printing guns are people with an LTC anyway, and the most frequently printed things are furniture and accessories. 3d printed guns are still largely a novelty, despite how much they've improved over the years. Even the much feared gun that Luigi Mangione supposedly used was bought legally, and any 3d printed parts were merely aftermarket grips or the like. The only large scale use of them that I'm aware of is in Myanmar, where they're using 3d printed guns to fight against a genocidal regime largely because they can get 3d printers and ammo, but no country is willing to support the resistance and so they can't get any actual firearms. You're much more likely to see a Garage Gun like the one used to kill Shinzo Abe, and those are completely legal by federal law - largely because it would be impossible to prevent somebody from just gluing a PVC pipe to a 2x4 and using a nail as a firing pin.

But firearms are so easy to obtain in so many states that it's much easier to buy one than to build one from scratch (whether that's buy one in the state or one with more lax laws nearby). There used to be a ban on gun stores within the city limits of Chicago, but Republicans got elected into office for like a decade and not only repealed that ban but also took the bite out of the gun laws, and now they claim that Chicago is proof that gun laws don't work when the city used to have some of the lowest rates of gun violence in the country. When they're not being bought right in the city/state, they're being smuggled in from the next state over with little concern for punishment.