I'm no lawyer, but I'd never have questioned selling personal property on a one-time basis myself, except that the initiatives and objectives of the ATF tend to change with administrations - not to get political, but any "agency" or entity responsible for effecting regulatory law has to work within limitations and are subject to assisting in vicariously fulfilling campaign promises - regardless of who's "in charge." I don't think the ATF knows what's legal sometimes and am not sure they would or would not have thought to redefine things established almost 90 years ago without some "encouragement" from higher up.
I personally HATE selling stuff online any more and have grown to really, really dislike going to gun shows, let alone setting up at one, so I mostly sit on stuff for a long time. My personal tack would probably be to sell by word of mouth as much as I can for SOMETHING more than the scrap value of the alloy, and eventually tire of that and convince someone to take what's left - the "whole lot" for a price they can't refuse. "Make me an offer and come get it!" The handling, dealing with carriers and their ridiculous prices, some nitwit who complains "there are water-marks on the blade" - wait, that was a knife. Done with that unless it's someone here I don't mind going to the trouble for.
Sitting on stuff has costs. At some point, that cost exceeds the value of what it is you're sitting on and, depending on the situation, costs and values are different, even subjective at times, but it's a personal decision whether one involves a field agent and a lawyer or not.
I've called the nearest field office twice within a year looking for some help on clarification on a matter and have still not heard BOO back, for what, like a year now? Yeah, that route was a dead end.
