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

Sonus Faber Stradavari. They do not come up often but when they do they are often in the mid to low 20's. They are a wide format, so you need some room for them. Glorious, from the old Sonus Faber line. But, then again, I am an old Sonus Faber junkie.

Just make SURE you buy from a reputable dealer .
There are a lot of sharks on here who sell one thing and deliver another.
 Agon will not help a screwed buyer in any way !
 A cesspool .

None, I already have the BEST; Golden Ear Triton two's, powered with a McIntosh 7270.
I would get something else like a Panamax 5400 surge protection, new turntable, Oppo 4K Blue Ray player and a few other toys.
Maybe some Audioquest Vodka HDMI cables......
I have FUN with my system.

I guess this is just the type of question that stimulates every "I-haven’t-heard-anything-like-my-last-speakers" speaker’s owner (just like me in fact).
How lucky you are to be able to ask such a question. And also to cope with the multiple answers everybody can read here.
I’d like to thank many of the gentlemen who took time to answer (and pleasure certainly), because I discovered a few names I had never hear of. Linkwitz seems to be that one I read most. I’ll pay a visit to them when I’m able to, for sure.
Let’s come to the most personal view, which is the speakers I would recommend in such a case.
Difficult choice, because, I must confess, I will mention speakers I never owned myself, unaffordable… I’ve been lucky enough to listen to them many times in my favorite hifi dealer shop in Paris (I’m French btw). I never owned them because they were (when new) and still are on the expensive side of my possibilities. But I’m only 54 so I still have some hope ;)
I’m talking about the wonderful Cello Grand Master (http://www.celloseattle.com/ctdocs/prodserve/speakers/grandmasters.html) for their ability to play anything with appropriate physical impact ; and the extraordinary Apogee acoustics Grand (http://www.stereophile.com/content/apogee-acoustics-grand#tG3tSR5EHGxwW1Wt.97) for their airy and incredible sound (the Diva can do too, and are easier to find. Depending on you room’s size).
I live presently with the babies, Apogee Stage. In a small sized european style room they are marvelous, driven by a Cello amp and Wadia CDP.
That’s an old-fashioned advice, I must admit. But your question is nice to be answered ;)
I wish you a nice way to your dream, that’s probably the funniest path in life, after the first kiss to a woman you love. Then I’m pretty sure that once you’re set up with the speakers you’ve chosen, you will start thinking about how you will ask here: « I’ve got wonderful XY speakers that actually do more than well. What would you suggest for the next step? ». Am I wrong?
;))