And that would apply to selling anything. Totally unrelated to gunsIn this case it would be capital gains. If it's income then you're in the business. What you have to report though depends on a lot of different factors. If you're to the point where an accountant would advise you to self report because you have obvious unexplained income, then you're way over the line with the ATF, too. To generate that kind of money from guns you would have to be selling multiple guns a day likely.