Most private citizens are selling their USED firearms. It's the selling of 6 unfired ARs of the exact same make and model that raises questions about "collecting" vs. re-selling for profit.
I have seen thinly disguised ATF agents appearing to be "interested buyers" quizzing walk-around sellers at gun shows offering multiple firearms.
The ATF will allow a collector maintaining a display at 2 shows a year for the sole purpose of liquidating inventory.
NOT a C&R collector, NOT a Designated Collector. Just a collector. No extra hoops.
No designation of unfired, fired, used, new or anything.
I've heard rumors that the kids at the Bureau of All Things Fun & Entertaining won't prosecute unless you bypass the 10-15% net income derived from firearm sales. But have found no guidance in the written law.