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

I must agree with Russbutton, above... very few speakers at any price can best the Linkwitz Orions or LX521s, when properly set up.  You get the sound of the best planar speakers with superb dynamics and bass.  And... my second choice would be the Pure Audio Project speakers, configured to your preferences.

Any of these driven by some of the better tube pre-amps (e.g.  ARC Ref 3, etc.), and some of the better SS amps (e.g. Pass Labs, etc.) - will provide you "best of show" sound, which you'll not be replacing any time soon.

Spend your money on attending live performances, and music... not on over-priced equipment.

Here’s a pair of the Linkwitz LX521’s with the ASP and multichannel amp - ready to go. I almost bought them... but... already had the Orions and couldn’t justify the expenditure for another set of Linkwitz speakers. Not sure the guy has sold them yet - you might check with him.

They sound like Maggies... but... with better bass and dynamics.  Same soundstage and imaging, because they're open baffle, dipole speakers.  I have the Maggies also... and... I prefer the Orions for the reasons mentioned.  Very much the sound of the live music performance.

https://www.audiogon.com/listings/full-range-linkwitz-lx-521-2015-08-02-speakers-07930-chester-nj

They also sound a lot like the Grado PS-1000e headphones mentioned above - which is spectacular... but... with a more realistic soundstage.

Here's what one Orion owner said about his Linkwitz Orions on the Madisound site:

"Orion 4 Madisound/Wood ArtistryThe most important thing I can say is that this is the best loudspeaker I have ever heard in my life for reproducing natural acoustical music, regardless of price. It's not bad for electronic music either. Pretty universally excellent on rock, jazz, trance, etc., but especially good for classical music. The most natural, nonelectronic sounding human voice reproducer I've yet heard. It is dynamic, crystal clear, coherent, has thunderous and tight lows, and most especially captures in a most captivating fashion the space of a live performace in a way I've never heard from a non planar speaker, although much more dynamic and with a greater sense of ease than a planar can do. I speak being quite familiar with a variety of ribbon/planar magnetic, electrostatic, and other cone (including ceramic) speaker designs. I freaking love this speaker!"

And the LX521s sound even better.

Regarding the Linkwitz speakers... even after hearing the various YouTube videos of them... I was a bit of a skeptic... until... I got my Orions driven by a good set up.

Now... I'm a believer!  

There are very few speakers that can match them (Orions or LX521s) - they do it all... as well as it can be done.

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.