I am not sure what it is. I have been watching some birds that I am interested in. In some cases they have been for sale at the same price for six months yet sellers are not softening up on the price.
So...my conclusion is that right now it is an Stubborn Sellers market. Most of these folks don't have to sell their bird...they should...they need to...but do not have to. So they sit for a year hoping to get their asking price when they could have dropped a grand or two and got it sold.
The problem is recognition of opportunity costs rarely go into aircraft sales decisions. Airplanes are as personal as guitars. Sometimes the reality of "market value" is greatly overwhelmed by either the thought of a loss or just sentiment.
I have sold two airplanes. Came out relatively even on one, and lost a bit on the other. But when I factor in the hours I flew and the cost of another annual, some other surprise maintenence and such, the grand or two I lost was minimal.
The problem with not recognizing minimal losses today, is that when a person really is forced to sell, their losses have the potential to greatly increase.