I absolutely do realize that. And in this case, he bought them LEGALLY, so it would have stopped that. At the very least, it would have severely impeded his insane goal. What good is you and I buying guns and ammo if we didn’t stop this? You and I buying guns and ammo never seems to stop things like this. Strange. (Not really.)
But yes, it would create (extend) a black market. Luckily, guns are not as easy to produce as say, drugs. A focus on the black market in this case would be significantly easier. Over time, that may change, but for now, it would work to great effect, I imagine.
By the way, I don’t think drugs (for the most part) should be illegal. I have no problem if someone wants to hurt themselves. I have a problem with people who want to hurt other people. My right to extend my arm ends at your face.