You are either assuming that he is selling to the black market, or you are not - but you are not making that clear.
No smart business owner would sell weapons on the black market, and risk losing his license and going to prison, so the answer would be - the same amount, the 5000 that you stated. Supply and demand for the black market would not change, as those individuals are not purchasing firearms from the owner. You cannot say that the legal production and consumption of goods regulates the black market - they do not share the same supply and demand. That's not how economics works.