Please pay attention. No one ever said that every person would obey the law regarding private gun sales. The law will be enforced a couple of ways: 1) A majority of the people who engage in these private transactions will obey the law.
2) Those who do not, and whose weapons end up being used in crimes will find themselves in huge messes. Let's say, e.g. someone bought a weapon and the sale was registered. Then that person sold the gun to someone else but failed to report it. When Ballistics shows who owned the gun, the registered owner is going to have hell to pay to get out of being charged with the crime (let alone civil liability for injuries). One or two of those will make people think twice about disobeying the law on this issue.

You know, this registration thing is a step in the right direction. Just because it is not 100% enforceable is not a reason to keep it from passing.
By that reasoning they should abolish speed limits on hiighways. They are not 100% enforceable either.