Buy a pair of ten year old $40,000 speakers for $4,000 or new ones at that price point?

The closest I have seen to this is speakers like the $40K Talon Firebird Diamonds that you can now get for $7K or so, maybe less, in perfectly good shape. They are every bit as capable as speakers made today, and will easily outperform speakers that sell for $7K new. Part of the reason they are so cheap is that Talon as a company doesn't exist anymore. If something goes wrong with them, you're on your own.

I recently bought a pair of Gershman Acoustics GAP-828 speakers, which are likely around 10 years old or so. Original list was around $15K, I paid $5K, and they were in perfect shape. One of the Dynaudio Esotec tweeters got a nice big dent while they were being unpacked, and I got in touch with Gershman and they sent me a brand new set. I also bought a set of midrange drivers to keep in reserve. This is why when buying older speakers, I think it really pays to buy from a company that's still around, so you can still get parts. I have one spare tweeter and two spare midranges ready to go just in case, and Gershman still has some of the original woofers left in stock should I ever need one. They can also be upgraded to the final GAP-888 spec ($24K list) for about $3800, which still represents a 66% off deal over new - and they basically would be new at that point since Gershman would put in new crossovers and new woofers as part of the update.

Good luck finding a speaker for $5K that can hit 22Hz like the 828s can, or that uses their "box within a box" construction, arced pyramid shape to reduce standing waves and diffraction, etc.

I have purchased  three pairs of used speakers used over the last 7 years.  All are in mint condition.  The first was a pair of Kef Reference 205's in perfect shape and perfect cherry finish. I paid $2800. They were a 'hold me over' speaker as I relocated, and I paired them with an Emotive amp and preamp.

When I got more settled, I found a perfect pair of B&W Nautilus 802's for $4100.  I think this was a great deal, and I upgraded (got my old) Classe pre amp and bought a used Classe CA 5300 stereo amp for $4800.  Since then, I have moved the B&W's downstairs and found a pair of perfect Revel Studio 2's for $7200.  This is just under half of their $16,000 new retail, but this is a current speaker and that is the best I could find.  I also now have BHK pre amp and BHK 250 amp, along with the Direct Stream DAC.  Wow.  For subs I bought new, but found a good deal on a pair of REL Carbon Limited.  I listened to several new speakers before buying the used Revel's; Kef Reference 3's, Proac K-36 and D48, B&W 802's, Paradigm and Vandersteen Quatros.  I did like the Proacs the best, but the K36's were over 20K.  I feel I got good value and great speakers for the money, and think used speakers are 'less scary' to buy used.  First, you can look at their condition and tell if they were well cared for.  Second, you can listen to them.

Here is my take on this.  The $40,000 speakers.  Have you heard them?  Do they sound better or worse than the $2000.00 speakers?  Never buy something sight unseen or unheard.  Also,
an important thing.  The drivers used on the $40,000 pair of speakers;  Can the woofers be
re-coned with the same material or if the surround is cracked or torn match the material?  Can you get replacement diaphragms for the tweeters?  If the mid-range unit is metal can you get replacement parts.(Voice Coils)   Unless all of the drivers are paper edge surround expect after about ten years that rubber surround dry out and foam surrounds rot.  Some tweeters have dampening behind the titanium / beryllium which can rot over time.  My point is that a new pair will be good for at least ten years and affordable to repair.  How long will the $40,000 now $2,000 pair last?  And can they even be rebuilt at a reasonable cost?

Typical cost on good used high end speakers is going to be much closer to 33% of original list - anything below that and you have to ask why.

I got a pair of B&W 802 Series 80 floor standing speakers from 1981 for nothing from a woman who was downsizing to move into a smaller apartment. She paid approx. $4,000 in 1981 for them, which in today’s dollars is closer to $10,000.
The dealer upgraded the midrange/tweeter heads so they’re technically Model F’s.
She drove them with an underpowered integrated amp with 12 gauge speaker wire in the 35 years she owned them. Knowing that the caps needed to be upgraded, I sent the crossovers to Walt D’Ascenzo, a brilliant audiophile who solved a ringing issue for Dave Wilson of Wilson Audio and shared tips with Walt Jung, the legendary audio engineer. What he sent back improved the 802’s to a much greater quality than they were originally. I’ve heard the latest offerings by B&W and have been underwhelmed. As a side note, with every tweak I make with better interconnects, power cords, and power conditioning, the speakers sound more fantastic. So when considering old speakers, don’t throw the baby out with the bathwater.