The term "manufacturer" is a term of art, with a legal definition. That definition can vary from state to state. In my state, there is a statutory definition set forth in the product liability act. It defines a manufacturer as "a person or an entity who designs, assembles, fabricates, produces, constructs, or otherwise prepares a product or a component part of a product before the sale of the product to a user or consumer." This seems to me to include you, the amature builder.
I was involved in a case in which an issue was presented as to whether a particular defendant was a manufacturer. This defendant had designed the original OEM gear in a reduction gear assemply in a military helicopter that crashed some 30 to 40 years after original design and construciton. The army took the plans and specs prepared by the defendant, and shopped the actual fabrication of replacement part gears to another contractor that had fabricated the actual gear that failed using the specifications that had been prepared by the defendant. (As an aside, the defendant had revised the the plans for the gear over time with increasing metalurgical technology, but the army didn't give the most recent updates. It just gave the old un-updated version of the plans to the new contractor.) The defendant contended that it wasn't a manufacturer, and therefore couldn't be liable. After all, it didn't fabricate the gear, had no knowledge this gear was being made, and received no payment for the sale of the gear at all. Someone else did all of that. The district court held that because the defendant had developed the plans, they were the "designer", and therefore it met the legal definition of a "manufacturer" set forth above. Consequently, the defendant could be held liable if that gear had been defective.
My point is that just because your common sense tells you one thing, a court may hold something very different. Where courts go wrong, I think, is improper use of syllogisms. A=B; B=C; therefore A = C. Except A doesn't exactly equal B, and B doesn't exactly equal C. So, you get "rounding errors" that result in decisions that look like 2 + 2 = 5.