If you're doing a small amount, it kind of falls under the "garage sale" clause. Nobody is really going to care.
Not really. "Garage sale" clause sorts of things, as mentioned by someone else, are due to the understood level of depreciation. Technically should still be reported, mostly no one worries about them.
However, anything that could not have undergone any similar level of depreciation, like re-selling ammunition, parts, or anything else----you are definitely supposed to report. No matter how little you do it or how little you made.
However, if you're doing it on a regular basis, buying and selling ammo to supplement your income, you *should* be reporting that income on your income taxes. Will you get caught if you don't? Probably not. However, that doesn't make it legal or right.
True whether or not it is a "regular basis."
That, however, is a completely separate issue (reporting income on your taxes) from the original comments, which were about the need for an FFL or a business license, both of which you do NOT need.