When a dealer sells a bare receiver he should always specify that it is a "receiver only", otherwise if he just puts down "rifle" it looks like he built it up into a rifle and sold it that way. Tax is due in that case, something he doesn't want. (OTOH, not all dealers realize this) Therefore it should be on his books as a rifle receiver, not a rifle. For the purpose of you building it into a pistol, you should be OK provided he specified receiver only and you verify with him that it was never built on and bought from a distributer/manufacturer instead of a private sale.
Reselling, trust comes into play. Once it is in your custody you could have slapped a stock on it first and made it a rifle instead of a handgun, thus a rifle forevermore. Something like an AKM receiver, it's pretty obvious if the rivets were ever installed, on an AR the buffer tube and stock could be on and removed with little to no evidence left behind.
Personally, I would never consider any rifle type receiver for a pistol build unless the chain of custody can be traced from manufacturer to my purchase from the dealer (or I made it myself) should the question ever arise. Safest course is to assume all receivers from private parties are rifles only. All the info an FFL needs from you is your personal ID info so he can enter it into his records.
Stripping a built firearm does not bring it's status back to virgin, all the rules still apply as to what was the first assembly, rifle or handgun.