So, for example, you have Rijas that builds complete guns based off lowers they purchase. Are they manufacturing or reselling?
Manufacturing.
I take a box of 1911 parts to my favorite type 01 gunsmith. He puts it together and cerakotes it for me. Gunsmithing.
I take a 10/22 off the shelf and thread it for a suppressor. Manufacturing.
I take a glock off the shelf and cerakote the slide pink to make it a "ladies edition." Manufacturing.