Exactly how is it illegal to build an upper for another person?

I have a 204 Ruger upper that I would part with. It's a DPMS upper receiver, DPMS 24" Fluted 204 Ruger SS barrel, DPMS gas block (Picatinny top), charging handle and round metal DPMS hand guard. This was originally a .223 DPMS Bull barrel upper. I swapped out the barrel a couple years ago but n...