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

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.

I will build you a cost no object pair of speakers , any design you want for $30 k . Top of the line drivers . get back to me if interested .

I'd only spend that much if I had a very large room to justify it.  

The he ones I would consider that might cost that much I know of are mbl and avantgarde and maybe focal or a few other large hi efficiency lines like classic audio reproductions. 

If not the Linkwitz Orion's or LX521s, or the Pure Audio Project open baffle speakers - which very few speakers can match... And I could only accept a "big name" brand (though pricey)... I would consider only the Nola speakers in your price range.  A superb semi- open baffle speaker line - which very few can match.