if a person buys a gun illegally, they are subject to arrest and conviction. Of course, we have sheriffs all over the country that have been mouthy about refusing to enforce such laws, particularly if obama is involved, and they need to be removed. Sheriffs do not make the laws, they are hired to enforce them.
Saying that because enforcement is hard, that a law should not exist is another red herring fake argument by the wingnuts. Weapons charges are used all the time to lock up bad guys, and has prevented untold murders.