So is denying the war crimes of WW2 Japan and recruiting people for an abusive sex cult!
Of course, Japan's so weird that when you assassinate an ex-PM for his links to a predatory cult the government says "good point, actually" and then does something about it.
Then it becomes an issue of intent, and how someone looking into you might interpret your intent. You might end up legally in the right but still end up harassed, jailed, and/or put on trial with all the costs that entails in the meantime.
If you're underwater financially or you're clearing out an estate sale, its a legal avenue, but I wouldn't make a habit of selling self-made guns.