What's going on with the used audio gear market?

It's inflation!  Used cars are up about 30%.

Market forces are in play. Basic supply and demand. 

+1 testpilot If they can get their asking price well good for them even if it is high. As you move up in the price range and the item is fairly new I can see asking more than 50%. I see ads for gear that is less than a year old that are asking more than 50% retail and I don’t blame them. For example ,let’s say you purchased a pair of speakers for $30K and after a few months you realize that they just aren’t working for you. If you sell them at 50% off retail you are taking a pretty substantial hit and someone is getting a pretty good buy. 
 

Anyone who sells below market value is a) not paying attention, b) not very good at 4th grade math, or c) an advocate for the greater good, unselfishly serving humanity.

Something else to consider is the replacement cost of those goods. If they are sold at 20% below current market value, the seller will not be able to replace them with similar goods at the same price. Thus, a very poor financial decision.

In past year I've sold some streaming devices at well over 50% price of new, lots of people getting into streaming, thriving market, especially at lower to medium end.

I've observed over the years some people ask high prices, item doesn't sell, no mark downs, item sits on market for months. I'm afraid some people very attached to audio components, emotional valuations don't compute for most of us buyers.