Well, it's much easier to catch bad guy owners than merchants, that's true. But most people don't really like operating in the black market. It's risky, your clientele tends to be assholes, and most importantly, you have to hide it. If you can be quite profitable in the white market, and the risk of loss from black market transactions exceeds the cost of due diligence, then we can expect to see very few black market merchants. So we have three options: Increase the profit of white market business, increase the cost and risk associated with black market transactions, or reduce the cost of due diligence. The internet is doing a pretty good job of reducing the cost of due diligence, and by removing restrictions on weapons, you would reduce it even further, and also increase the profit of white-market business by moving more business into the white.

Hey, imagine that. Fewer laws, fewer criminals.