Won't matter - people will never obey the law and there is no way to know where all the guns are. Even if they are registered to you - you have the right to sell them if you want. The legal requirements to sell a gun in my state are to make sure the buyer is also from your state and that they do not have any legal status that precludes them from owning a firearm. This can be just a verbal statement from the buyer. So good luck finding them.