Why a dearth of some brands on the used market?

It had always been on my bucket list to own a pair of Luxman M-600a amplifiers to use as mono blocks. They are by far not the most expensive or coveted Luxman amp, but they rarely show up used in excellent condition - seems people are listing ones that have a good amount of cosmetic issues. They have been discontinued for years, and only a small population own a pair. A single in stereo mode is 30w of Class A into 8, 60w into 4, but in mono BTL mode each amp becomes 120w of Class A into 8 and 240w into 4. I also like their form factor and aesthetics than any other Luxman piece (all of which I regard quite highly).

I finally found one and purchased it while I’m really happy with the performance of it, I can see where a second one would deliver the added control and resolution I am missing against my reference system. So I’m patiently waiting for another one in excellent shape to come off the market. It may take a year or so, but I’ll wait.

I also agree with @yesiam_a_pirate regarding quantity produced. Also, there are different types of products made for different types of audiophiles. Not all audiophiles are gear swappers, but those that are tend to be attracted to brands that target them. Those brands also sell a large volume and then release new, upgraded products often, which drive a percentage share of consumers to sell their older ones off. 

Dyns are for sale because many customers keep stepping up the ladder.

I heard the Devores directly after listening to Dynaudio and Harbeth and was totally underwhelmed after all the hype I had read. May have just been me.

Maybe people actually like them and keep them.