What is the best USED Speaker Under $10,000

Without a doubt, JBL 4367.

Simply put they are state of the art, amazingly designed and engineered. 

Why buy used when you can get new but sold direct, saving sealer markup?

Contact Emerald Physics (sold direct) to see who has their 2.8s in your area

2.8s received an Absolute Sound award

MSRP $9998, but they run specials,

I have the KCIIs with upgrades similar to their Pro model. One of the veery best speakers I've owned, including Genesis Vs, Accoustat 2 + 2s, Nearfield Pipedreams, Magnepan 3.5Rs

KCIIs < $2500    

I just went active with a pair of DSP8000’s so the big ushers are going on the market, sniff. Then I’m done being tempted by the next best thing.

I was on the same hunt about a year and a half ago. Did a little traveling and listened to maybe a dozen pairs of speakers. I ended up with the Aerial 7t’s and couldn’t be happier.

The direct sales approach may save you money or it may not.  It's still all about how good it sound sin your system/room.  Many of these companies still are pricing their products where they would sell if done through a dealer network.  They just make a larger profit margin compared to those sold in stores.  Not picking on EP at all, but making a blanket statement that it's not always the case.  Psychologically many feel they are getting the bargain of their lives, but it still comes down to sounds best.  

If you are a good shopper, you can easily find a great deal used.  There are many companies who don't have as large a dealer mark up as say a Wilson or Magico and are also a great deal new.  No absolutes in audio as we know.  

Interesting conversation though.