Scenario: I order a new Daniel Defense DD5V1 at my local GS. They in turn order it from a wholesaler or direct from Daniel. The paper trail starts. I do the form at my LGS take it home, and wife has a total fit. I sell it to you without paper work for $200. You in turn sell to someone else with no paperwork. The DD5V1 is stolen while the new owner is out of town. Some mentally ill person buys it on the street and shoots up a church. The nutcase escapes the crime scene. The trace starts. I have a bus load, the entire alphabet of federal agents breaking down my front door with a warrant. I tell the cops, I sold it to some fella named mac from the internet. Before a carpenter arrives to fix my door I have TV trucks parked in front of the house. Mac, I did everything within the law. So did you in this scenario. My face is plastered all over CNN. Nothing but trouble. I insist on a bill of sale. Do as you wish.
And can you point to how many of these scenarios have actually played out? There are thousands of pvt sales all over this land and I dare say most dont have this fantasy.