From various conversations and posts I've had / made over the past few days regarding gun control and background checks, I'd like to offer this observation.
Point Of Sale is the wrong time to prevent criminals from obtaining the tools they ply their trade with. If you wait until they try to purchase something before you figure out "Hey! They shouldn't have this! They might hurt someone with it", then you've waited too long to be effective. It's like trying to put your seatbelt on while slamming on the brakes.
You limit a criminal's access to dangerous items by incarcerating them, not by hoping a cash register somewhere properly identifies them and refuses to make the sale.