And its easy, all you have to do is mail the ATF a letter requesting it be removed from the registry. Same place I send my prints for an F1 or SBR submission I believe. Needy Road in WV from my memory?
Me taking the collapsible stock off my stamped AR-15 SBR and selling it to any individual via an on line ad is pretty dumb imo. (How about out of my car window for $500 cash? A drive by sale, like purchasing a daquiri or a hamburger.) Also engraved with my name and city/state. Without taking it off the NFA registry first. I mean sure I could grind away the engraving first, but its SN is recorded/registered too. De-facing a firearms SN is a no no too. But we who possess stamped NFA items have already played by the rules for compliance, so why would we NOT send the letter first?
I don't recall seeing where the letter was required, only that it was a good idea.