I own (and occasionally buy and sell) mostly 'vintage' audio equipment (1960s - 1970s), so my comments may not be 100% transferrable to the more modern audio community... but, IMO:
I do feel like we MAY have now passed the 'peak' in terms of getting top dollar for used equipment. I think many of the factors mentioned by other commenters are valid partial reasons for this (current economic climate, large increases in shipping costs, ease of comparison shopping via the Internet / more people 'flipping' and selling, etc.) but I also feel there are larger forces at play.
I think the main reason for the decline in the modern used audio market is that modern audio enthusiasts put more of a premium on having the latest technology; a lot of their interest in purchasing equipment is to access new tech and new features. And even 3 or 4 year old gear might not have all the latest tech and features. So they skew more towards buying brand-new vs. used right off the bat.
For the vintage audio sector, interest in general is more driven by nostalgia and not as much by the technology itself... middle aged (and older) guys like me who either want stuff they had when they where younger (or a parent or relative had) or they always wanted "back in the day" but couldn't afford it back then... the main reason for a decline in vintage is simply - and bluntly - the main pool of hobbyists are dying off, and there's not a lot of younger people filling in the ranks (most younger people, if they have any interest in audio AT ALL will gravitate towards the modern equipment with the technology they are more familiar with...)
The other main problem in both audio communities (IMO) is that sellers are often overly optimistic about setting a price point. They look at listings on places like eBay, Audio Mart, Reverb, etc. and often times feel justified setting their own pricing based on the highest priced listings they see... and many times that listing has been sitting there, with basically zero interest, for months or even years. One obvious thing is to filter down to the actual "sold" price for the same equipment; as there is often a large difference in what something is listed at, and what it actually sold for. I also suggest looking at how long the item was listed (when that information is available) so you can decide on perhaps a balance between what you want to get for your gear, and how long you want to have to list it for, answer messages, send demo videos or host in-person demos for, etc...
Another important factor is you need to price for the local market you are in. With shipping becoming so expensive - and horror stories of buyers receiving damaged gear on their doorsteps becoming easier to find and read about - more people are shying away from buying expensive, heavy electronics that need to be shipped to them. I don't sell a lot of equipment, but I have noticed a trend towards more and more of what I do sell being bought locally. And those potential out-of-state buyers being a lot more concerned with shipping details such as how is the equipment being packed / who is packing it (DIY or a professional shipper); which shipping company am I going to use; details on what my 'policy' is if something arrives damaged, etc. Honestly, for me, losing a little in sale price because it's a local sale and not getting shipped to LA / NY / Chicago for top dollar... is often offset by not having to hassle with packing and shipping - and the buyers who still insist you can ship a 60 pound piece of audio equipment across the country for $50, so why I am I "ripping them off" when I quote $150 to (professionally) pack and ship my unit..?
