The vast majority of these 3d printed guns use the fact that the registered “gun” is only a small part of the actual whole gun. You can buy barrels, magazines, slides, firing pins, etc, all mail order without any fuss — it’s the part with the serial number that is actually the firearm and restricted. So if you print that part (which is often basically just a shell to hold the metal components in place) you have an unserialized gun that’s basically just as good as one from a gun store.