What used speaker would you buy with $20,000-$35,000 and why?

I have attended the annual CES for about 35 years. This year my vote for the best sound goes to the Magico S5. The imaging must be heard to be believed. I have seen a few used S5s on Audiogon. I would strongly urge you to have an audition if there is a dealer near you. 

I have no qualms with the other suggestions as they are all good but Magico has my vote.

Try the JBL LC310-1 speakers if you can find a pair , I bought mine for 50 bucks

We at Sunny Components have several ideas...

Eggleston Works Andra 3
Wilson Audio Sasha Series 2
T+A Elektroakustic TCD110 S

Did you ever heard of the BIOSONIC MACH 2 (by Jean Philippe MARTIN) ?

What speaker?  Hello can of worms.  If you are spending 20k on speakers I pray that you go listen first.  That's a lot of money and there are some nice choices.  It all depend on your tastes, the way you hear, your room, gear etc.. 

My personal is Vandersteen. You can buy a new pair of Quatro CT's and get the latest updates etc...  Nearly everyone I've steered in this direction has been MORE than happy.  As you probably know it has tunable bass so it interacts with most any room extremely well.  I use the Treo's and am putting them up for sale as I want the Quatro tunable bass.  

Unless you could find a pair of used 5CT's from Richard, I'd just go get a pair of Quatro's and use the extra to buy music or upgrade your amp or music server (Melco is a wonderful server/NAS that's easy to use and sounds pretty awesome).  I'd also get a double pair or AudioQuest Castlerock speaker cables and do a true bi wired run.  You'll be very very pleased you did and you are still probably under your 20k.  

BTW, Vandersteens sound best with a zero feedback amp like all the Ayre products.  They also are awesome with anything Audio Research or Aesthetix if you are into tubes.  

Where do you live?  I'm sure there are stores near by to go audition.