Why bother with regulating firearms sales when the black market is up and running for the persons inelligable for legal posession?
Because it's a lot easier to prove that someone has a gun than that he committed some specific crime.
I think it's something like the three strikes law. If you've committed two prior violent crimes, you can go to jail for 20 years for shoplifting bubble gum.
Some say that's excessive, but I'm not bothered at all.
If you've proven, though past behavior, that you cannot be trusted, you're put on notice that we will not tolerate behaviors that we accept in others.
If you're a released felon, we don't forbid you a gun because we think that we will prevent you from obtaining one, but so that if we ever catch you with one, we can throw you back in jail, without having to wait for you to shoot someone.