Mary Jane Humphrey, Louisville KY ATFE told me this, a stripped lower is sold as a stripped lower. Selling it as either a rifle or pistol MAY be a violation as there is no way of knowing ahead of time how it will be assembled. It is how YOU assemble it first that makes it a rifle or pistol. The NICS check means nothing as that check is not tied to a serial number.
So, get a stripped, never assembled lower, assemble it and document it and be done with it.
The letter shown on the above link in good, but the situation is that unless you get something in writing addressed to you, it is meaningless. And just trying to get the ATFE to respond on letterhead of late is an exercise if futility. I got my e-mails from MJH so that is going to have to do for me. My letters to the ATFE have gone unanswered for over a year now, and that amounts to over 24 letters. And how can a manufacturer know how the lower is going to be assembled?
Also, as an SBR, you can always have it removed from the registry. You need to contact the ATFE but I think it goes like this. You convert to non SBR status, then send the ATFE a letter telling them that you have converted it back to non SBR and want it removed from the registry. Then after you get back your letter from them acknowledging such, you can sell it as a non-SBR.