For curiosity sake, would a used lower from an individual cause issue? They are so inexpensive that I would probably not buy a used one, but curious anyhow.
Well, was the used lower used as a pistol? If so, no problem.
If the used lower was a rifle, there are restrictions on making pistols from rifles.
You could just get a lower, engrave and form 1 it and forget about wasting time with it in pistol configuration.