I think there are multiple issues all culminating into what can be happening.
Brand stability issues, like mentioned above with Audio Research can have a large negative effect on a particular brands secondary values. Owners want to ensure there is service, support and parts available after the purchase down the road. Especially since this is an expensive hobby. The opposite side of the spectrum may be a company like McIntosh. If the glass on a 50+ year old amp/ preamp needs replacing, or if the unit needs servicing, the manufacturer still can provide those parts and services. That carries a lot of value to people.
Shipping costs have gone insane the last 5-ish years. What would cost $60 to ship 5 years ago is double that. Some larger/ heavier gear can now be several hundred dollars to ship more than a few states away. Buyers take that into consideration, as well.
Some gear is just more commoditized, like DAC's, for the most part. Those things are being churned out by everybody and their brother. Stuff like that just lowers the price in the secondary market incredibly fast.
I'm sure that are more issues, as well.