It is a supply and demand problems at it's simplest form. The supply of old guys buying doubles to hunt with, shoot or collect is drying up. Several mid to major collectors I know are done or worse have passed away. Dead men buy no guns. The number of double guns to choose from is increasing as old guys sell off their stuff. This combination of too few and too many is keeping prices depressed and will do so for the foreseeable future.
And it is not just doubles that are hard to sell. Pump guns, other than a few select ones, are dogs on a gun rack these days. You can sell high condition Model 42, 12 and some of the 37's but the vast number of well used guns are just impossible to sell in most shops. If not in great condition my favorite gun shop will not even accept them on consignment. They might as well be bolt action shotguns these days, and you can just imagine how easy those are to sell.
And there is no hope for the younger shooters suddenly becoming a major force in the market. Young people are more into black guns, semi-automatics and camo than doubles. The non-tox shot forces them to buy those guns even more so as three bucks a shell is too expensive for most hunters. The need to change chokes for different game or games make the screw-in choke tube guns a way to go for many. Hunting is getting to be prohibitive expensive for most. And I think the taste fir wild game is in much decline.
So it has become a real buyers market. Figure out what you want and it is out there at your price if you look long and far enough. I do not even worry anymore about missing a bargain gun because there will be another one along soon enough. Rare and extremely high grade or high condition guns will sell for a premium but run of the mill stuff will not and junk will become just about impossible to sell. Supply and demand working for and against us depending on your perspective.