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

I definitely agree with Almarg & rzado re. seriously auditioning (with a preference to buy) a Sound Lab ESL speaker.

whatever you do make sure that you get a time-coherent speaker (most of the speakers on your list are not time-coherent). The Sound Lab speakers are time-coherent. You mentioned Vandersteen - yes, these are time-coherent too & a very good choice.
I would also like to add to your list Green Mountain Audio - if Roy Johnson (owner) is still making their 3-way Calypso then that speaker would definitely be on my short list.
And, of course, i will highly recommend an Apogee speaker - Scintilla 1-ohm version. Nothing short of the best. You have an amp with cojones to drive a 1-ohm load so no issues there. 

In my experience time-coherent speakers sound the very best - realistic sonics, hours & hours of non-stop listening, zero listening fatigue, very long-term enjoyment, every CD/LP sounds great & as the electronics gets better, the music playback gets better. Whenever I’ve heard the Green Mountain Audio speakers in my room (when i owned a pair) & at audio shows it always sounded like a live band playing in the room.

OP's question is mildly amusing on many levels. The thing is, no matter how much one spends on speakers, how does that correlate to the basic notion of simply enjoying the music?. 

Is 20 +K some sort of arbitrary threshold?  My daily driver (aka car) cost less than 20K.

Does a "previously owned" 20K speaker do the job?  Or do you need to spend 25K?  And,  what if you expand your budget and include used 30K speakers?  What if you go the "cheap route" and buy speakers for under 2K and put the rest of your  money into a retirement account or donate it to a worthwhile charity?

Audiophiles... god bless 'em. Or not.

Here's to PS:
OK, I've been planning to respond to a bunch of the responses before this one (with mostly "thanks" and noting the good advice), but his/her last post is mildly offensive on many levels. So "thanks" to the thoughtful recommendations you folks have given - I appreciate it. I will need to go on a bit of a listening tour in the near future. 

Congrats on your 20k car. I don't know what proportion of your net income and/or wealth that amount represents but, for the sake of argument, let's say it represents less than .2 percent (that's not 2%, that's 2/10's of a percent) of my income and that I'm considering a 20k used speaker as a responsible use of my money in enabling me to be "simply enjoying the music" and to do good for other folks. More concretely that is the equivalent of an individual with a $100,000 income spending $200 on speakers.

IOW's I could go out and buy speakers costing much more because I can afford them, but I would rather buy used so I can fund an orphanage that I started.

FYI my retirement account is fully funded. I may be wrong, but I'd bet I give more to charity in one year than you have given in your entire life. The "reductio ad absurdum" argument would ask, "Why buy speakers at all?" Divest thyself of all things and help the poor (who you will have with you always). We should all become monks and chant ourselves into music bliss.

On a practical level the question revolves around getting the most amount of sound out of high end used speakers for the price. And doing so after one considers that some of those speakers hold their value more, and others depreciate more rapidly in the secondary market. The former may sound better in the original retail price range and yet later, once brands which entered the market new as more expensive price points have depreciated to extreme levels they match the price of the brands that hold value, it may be there is a "flip" in the price/value equation.

Some lessor known brands really drop in the used market and become substantially better values ($ per sound unit) than do units that really retain their value. That was the thrust of the original question. It was not meant to tout my wealth. Which is less than I've stated (but I'd still bet that I give more in one year to charity than you have in your entire life).

Heck I don't drink Starbucks coffee, but I can afford it.

Audiophiles . . . God bless 'em. Non-audiophiles . . . God bless 'em too. Stop the blessing limitations, He does.

Great answer.  I agree with value per dollar, however being audio, the real question becomes which speaker will sound BEST in your system.  When I said Vandersteen or other names speakers will keep their value, that doesn't mean they still won't be the best buy for you.  Taking into consideration that they well may be the best sounding for you to begin with.  If so, then that's the speaker for you to get I would think.  The fact that they will cost less in the long run if you need or want to ever sell them again in the future.  The bottom line is that for this kind of money there are so many good options.  You really need to drill down to see which speakers you want to audition based on what's around you to listen to and if there is one or two reviewers who seem to listen the way you do and appreciate what you like to hear.  That way you don't go crazy like I did (although I'm glad I did and it was a fun two year journey getting to Vandersteens, which I went into my search not wanting at all).  Companies change their sound greatly over time with new components to work with and new parts as well.  I listened to so many speakers from the 90's to current and most of the speakers who's parts are over 5 years old, on the whole, didn't sound as good as more recent designs etc...  JMHO.

I'd travel to CA to visit Teresonic and listen to the Ingenium. Why? Because my next stop on this quest will be a true single driver speaker system.