SBR or pistol?

[DAS HUGH!]

Pistol first, you can go back and forth.
Rifle first, always a rifle(without nfa paperwork)

FFL should transfer the receiver the same way it came in. Ie don't call it a "handgun" if it came from manufacturer as an "other"

As a retailer of used guns, how do you process a mystery meat used AR typically? Do they make you dig and find out what it is, or was? Or do you sell it and call it by what you see in front of you?
For instance how do you know if it isn't a rifle lower that someone put a brace on pistol upper onto, before you turn around and sell it?