Your search is too large. On Gunbroker, almost all gun sales are for no reserve auctions. By a ratio of more than 20:1. So if you want to see what guns are selling block out all reserve price guns. Then do a high number of bids search. That will give you the guns acutely selling at auction. All the rest are just dead items, listed endlessly or so over priced that they almost never attract attention.
NIDs do sell there but for $300-500 for all but the highest condition examples. Small bores for more, but not that much more. So any NID listed, with or without a reserve, with a price much above $500 will linger forever with no interest. Winchester 21s all seem to have a starting price about full retail. I find that most buyers will take a chance on a gun for a few hundred bucks but want to personally examine guns before taking a chance for thousands of dollars. It take a great looking 21 to even get a bid unless it starts out very low.
The problem is that sellers seem to want retail to high retail prices and buyers want only half price guns. Not a lot of overlap between the two. Then most buyers have figured out reserve auctions have the reserves set at full retail so there is no point even bidding on them. Hence items get delisted forever and never move.