The receiver doesn't need to be labeled pistol. When the dealer sells it, he notes it as just a recevier. As long as it hasn't had a stock mounted on it, you'll be fine.
quite a few people selling new lowers from their "private collections", so there's no paperwork involved.
If it's done at a gun show, there must be a background check done regardless. Gun shows constitute 3 or more people viewing firearms in one location.