My guess is the Wholesalers, most will buy in quantities at low prices, then mark'm up. If their mark-up is too high, retailers won't buy from them, and the item sit's there.
Then X years later, the wholesaler see's he has a random mix of items left in his warehouse. He decides a smaller mark-up or break even cost, ( let's say they figure cost of warehouse space @ $20 per year per square foot), mixes a bunch of random stuff too sell as a "lot", and someone buy's the "lot".
Viola',, the gun hit's the rack
Or a small Mom-n-Pop's could do the same, they have 5-6 guns that haven't sold and offer them up as a lot too get their money back. An they get shipped to another region.
We have a small sporting goods wholesaler next to our shop in the industrial park.(50,000sq.ft.(?) Hunting, Fishing, Camping,,all kinds of stuff. A buddies wife works there, they turn over rifles, pistols and ammo. It's interesting the security they are required to do, special dock door, caged area in the building, etc,,,
Anyway's they buy rifles in groups of only 5-10, and they get shipped "peice meal" to several local small shop's, not the box stores.