Who said anything about lawful? And, furthermore, it is not unlawful in most places to do a private sale between two individuals.
I cannot see anything you said that answers how "they" will know if I do not register a firearm that no one knows I have. This is exactly the reason that registration will not reduce crime substantially. Those who perpetuate crime with guns didn't buy those guns through a means that required a record of the transfer.
Who is going to know if guns are in an estate, unless the heirs tell the authorities? No one scrutinizes property that is inherited.
Buying ammo that is "specific only to that gun"? What does that mean? Where do you go to buy ammo for a one of a kind gun? Are you saying an ammo dealer will have a list of every person who owns a .308 W ?, for example?
I repeat, no one can ever do anything about guns they they do not know exists. Officer says, Joe Blow says he sold you a .458 SOCOM AR15, about 8 years ago. Answer, "He's lying." or "I resold it to a gypsy.", ad nauseoum. Who can know one way or the other? And, how about all the millions of them that no one will ever admit to even having in the first place so that they COULD resell? No way, craig.
SRH