In answer to your query, Rock Island Auction.
We drive Volvo XC70 wagons ordered and purchased new until the junk yard.
But I don't purchase and sell with the intent of making money nor losing money.
I purchase what I want at a reasonable price for a collector grade gun and use them. After 30 years of this, I simply got tired of the Collector Market, the crap I had offered and crappy business ethics and moved to custom(bespoke) guns.
As pointed out the market has narrowed and in the case of Winchester branded Model 21's, won't be increasing very much in the future. Note, that I said "very much". there are probably 21's still be cobbled up out there by individuals and records inserted into Cody.
If you get tired of the used Model 21 market,
CSMC makes a very nice Model 21 as good if not better than the original.
I own a 'Baby' frame 28 Grand American and am in no way worried about resale.
It's simply the finest Model 21 Winchester never made!-Dick
