Everybody is correct - FET is only for licensed manufacturers "in the business" of selling their product for profit. This is taxable just like any other manufactured product, and justification is probably the interstate commerce clause in the Constitution.
However, none of the above means that
a) You are limited in the number of guns you build for personal use
b) You are prohibited from selling any gun you make, as long as you aren't doing it as a means of generating substantial income.
c) You can't make any kind of profit when selling a gun you made for personal use. For example, suppose you buy a bunch of parts and make an AR, then some kind of ban is enacted, making your gun more valuable. There is nothing requiring you to sell it at the old value rather than the current value as a "pre-ban" gun.
You would only have a problem if you regularly made guns and sold them, and the sales generated "significant" revenue to you above what an occasional sale would bring. Note that this is exactly the same thing that would apply in selling any gun you own, or even certain other things, like cars.