How many times do we have to have this conversation, the rules and laws are actually very clear, if you build it as a rifle, then it is always a rifle, if you build it as a pistol, then you can go back and forth..Now a days, stuff is so cheap, there is no reason to even worry about it.. Keep a couple of pistol configured lowers in the safe..
As far as buying a used stripped lower receiver, then you should side on the side it was a rifle! if you want to build a pistol, order a damn stripped receiver, put a pistol tube on it and take a picture of it where the serial number is visible, if you are doing 80% I don't know what to tell you, if the receiver was built by a manufacture, then when it was sold, it was listed as a rifle and that description does not go away, it is recorded somewhere, if it is a stripped receiver, then if they did their job correctly when they sold it, then it is other, if it is a 80% then there is NO record of it, it is a paper weight, period!
This is not complicated, it is clear if you have the capability of comprehension.
Come on guys!