Well, it starts with enforcing law. When you say loud and clear to somebody if you're going to carry a gun illegally, we're going to arrest you. If you're going to sell a gun illegally, you need to be arrested. If you commit a crime with a gun, there needs to be absolute certainty in the law.
According to the law, it is ok to sell a gun illegally
( z1 okay :ARG1 ( z2 sell :ARG1 ( z3 gun ) :ARG1-of ( z4 legal :polarity - ) ) :ARG1-of ( z5 say :ARG0 ( z6 law ) ) )