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

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).