*edit*
Even if they did sell a registered firearm. What's to stop them from saying it was stolen?
well if they do that more than once, it should raise a red flag. if every time you buy a gun, it conveniently gets stolen, then obviously you are not being responsible with the weapon
just because there's ways around something, doesn't mean you shouldn't take steps to prevent it, or at least make it more difficult. the analogy I usually use is that you lock your car door, even though someone could just break your window
edit: it comes down to the question of if you want to do something, or nothing, to keep track of the guns out there. "nothing" means a free-market where anyone can buy and trade guns. I don't think many people want that