What are the speakers you very rarely find in the Used market(Audiogon or Anywhere)?

I have a old M&K sub

There’s a good chance that I will be selling my Vandersteen Quatro CTs within the next couple of weeks, for local pickup only. Stay tuned...

Vapor Audio.

revel f52.

My Quatro Wood CTs are now for sale, just not here.

McIntosh XR-250

Rethm....

MAGICO Q3s the best.

Local Craigslist has these at a great price, not mine.
Aerial Acoustics 10T audiophile speakers - $2,400 (Newbury Park)

Sony SS-AR1 or AR2. 

You could try deciding on a couple speakers you are interested in and run “wanted” ads. I did this for a pair of tube friendly, fairly rare speakers and was contacted and bought them with no competitive offers. The only problem is that this took almost a year, so if you’re in a hurry this isn’t an optimum solution. You never know though, could work out quickly.  

Agreed with some earlier people.  For popular speakers, the Devore O/96 are rare and go fast.

For less known but great speakers that are very rare in the used market:   Waveform.

Upper models from Martin Logan (e.g. Spires and up).