Why is used audio equipment so undesirable?

Do diligence. The only two things I bought used and kept was a pair of Sennheiser 595 ear speakers and a Yamaha T-70 tuner. Both arrived safe and sound and still work  A Yamaha C-4 preamp stopped working through the speakers w/i two weeks, though it did work with ear phones. A guy who repairs Yamaha equipment took it off my hands. I lost a nominal amount of money.  Buying used can save you a lot of money, but it can also cause you much grief. Know what you're buying. 

Due.

I've started buying used from dealers and trading in old equipment to get better price reductions. Selling MIT cables and a BAT tube amplifier brought me to that conclusion.   The ads for the gear had reasonable prices based on market research, but buyers negotiate much lower.  I don't know what I'm going to do when I eventually sell everything.  The equipment I enjoy isn't desired by a lot of people (I.e. MIT, Spectral, EglestonWorks).

Prices are rising for Vintage, maybe there is a hole in the middle, used/not old that you are dealing with. I sell my used equipment. Money helps me buy 'new to me' old stuff, or new stuff.

I work hard getting it in best condition, get documents photos descriptions, links to OEM info and reviews. That makes a difference. Make the effort, it will go.

Took my 2 McIntosh SS pieces to McIntosh, had them factory certified, new shipping boxes, did very well, easy safe shipping.

Current Fisher 500C, Excellent condition and sounding, many fisher oem tubes. I overhauled it's controls, dead silent, had VAS align the tuner, last step: brass caps need to be polished, then Steve at VAS may buy it, he wants 1st right of refusal! No wood enclosure, will be local pick-up only. That's limiting your audience, however I'm midway between NYC and Phila and can meet someone half-way, so we shall see. Could ship, but the alignment just done would be worthless.

Sold nice Carver Amp, Carver Tuner, Onkyo CD changer ... More stuff on the shelf, I found my SME 3009 II, needs some care, then sell.

IOW, you can do alright, but it takes effort.

As a result of reading through each of the comments I have come to some revelations and they are as follows:

Rather than run the risk of having used gear for sale always stretch to the highest quality you can afford to avoid the costly upgrade cycle. And maybe you'll keep the component until death do you part.

Avoid buying used gear to avoid the constant thinking where u wonder if it would have sounded better if u bought it new.

If I trade in used gear I will likely stretch my budget to buy something even better. 

Maybe I keep the gear and eventually have enough to set up a third listening room that I will never use but it will look nice.

Finally, Selling or trading in used gear Will serve too only reinforce that u lost a lot of money on gear that you didn't have for very long.  Better to store it away to forget about it or give it to a family member who otherwise would never spend crazy money on audio gear, and Hoping they don't talk to you about audio gear whenever you get together and avoid the insanity of wanting better gear and spending a ton of money.