"Manufacturing" is one of those things that has a statutory meaning different from the normal usage.
Because Congress' power to license firearms comes primarily from the Commerce Clause (but in no way involves the 2nd Amendment), it has to do with business licensing.
Definition of Manufacturing, in terms of needing an FFL, from ATF Rul.
ATF Rul. 2010-10:
"ATF’s long-standing position is that any activities that result in the making of firearms for sale or distribution ... constitute firearms manufacturing that
may require a manufacturer’s license. [Italics mine]
Although installing parts in or on firearms, and applying special coatings and treatments to firearms are manufacturing activities, the definition of "manufacturer” in 18 U.S.C. 921(a)(10) and 27 CFR 478.11
also
requires that a person be "engaged in the business” before the manufacturer’s license requirement of section 923(a) applies.
Thus, a person who manufactures a firearm will require a manufacturer’s license if he/she devotes time, attention, and labor to such manufacture
as a regular course of trade or business with the principal objective of livelihood and profit through the sale or distribution of the firearms manufactured. [all italics mine]
So, if you collect and/or tinker with guns and sell a few here and there, you're fine even if you put the funds "back in the kitty" so to speak.
But if you serially buy/build/sell/reinvest and that's all you do, you are essentially dealing in firearms and you will need to get an FFL to be legal.
And before y'all start in about jackboots, bear in mind that the exact same thing would apply if this was used cars instead of firearms.
These guys who always have two or three cars for sale in that lot by the highway are always getting busted by the state tax people, and they really nail 'em.
(Speaking of state laws - as someone said above, know and obey!)