A law is useless when the law:
1. Cannot be enforced
2. Is set for people who are already breaking the law
3. Will not be able to be tracked
4. Can easily be worked around legally
Details:
1. Cannot be enforced. How are you going to prove that X person bought XYZ gun from Y individual? It's not like they give them a receipt. Law would be unenforceable.
2. Is set for people who are already breaking the law. People on the black market selling to criminals are already breaking the law, and the criminal is breaking the law by possessing the gun in the first place, so adding yet another law they are breaking won't do anything. Remember, the goal isn't to have as many laws broken as possible, it's to protect kids. Remember?
3. Will not be able to be tracked. This kinda goes back to number 1. With no records, how will you do this?
4. Can be worked around easily. What stops someone from GIVING someone a gun? And then I change a lightbulb in their house and charge them 250 dollars. That's completely legal. Without gun registration, it's pointless to force FFL dealers to do background checks because nobody will know what happened and when.