Buying a finished receiver and assembling a complete rifle or pistol is NOT manufacturing. There are no more restrictions on assembly of a legal configuration weapon than on buying complete guns. Only major exception being import parts may be restricted to a certain number.
If you assemble a number of them and sell them for profit, the BATFE may nail you for "engaging in the business" of selling firearms, not manufacturing.
Manufacturing means building with intent to sell. Only manufacturers need a license to build a gun.
Building a gun for yourself is NOT manufacturing and needs no license as long as the gun you build can be legally possessed by you. The BATFE has, somewhat arbitrarily, decided the difference between assembly and building is determined by who does the last (at least) 20% of the work to finish the receiver. That's why there are 80% receivers available.
If you build it and get tired of it, you can sell it, just like any other legal gun you might have.
There is no federal restriction on how many guns you can build in a year.
Don in Ohio