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

I guess I would look at the problem your seem to be trying to solve in a different way.

You have $4000 to spend, OK.  What is the size of the room, what electronics to you have to power them, what are your musical tastes all must be considered first.

Then you can look at what you can get new for 4k or used for 4k that meet those needs and make a decision.  Certainly there is always some risk buying previously owned but the site has a thriving clientele buying and selling "used" gear.

Also new speakers are easier to audition but even if looking at particular brands used, you may be able to listen to the general sound/voicing from that brand to help.

If I were you I'd take that approach, put in the time and effort and not expect a silver bullet answer 

 My 25+ year old Legacy Focus speakers are still being made, with higher quality parts, for $13,000 while used older pairs go for $2-3,000.   However, the older units sound better.   Same with the Legacy Signature IIIs, selling for $1-2,000 (foam surrounds on woofers need to be redone rubberized).  New Signatures are about $7,000.   So, not 10X cheaper but very significant bargains in the used category for great speakers.

Big heavy speakers are a challenge to sell at any age. So when someone has to move them, bargains are possible. So key is listen to them and assess their value.  But speakers do need foaming when they age, and that could be a problem. Also are parts available, is the company in business still, etc? This would greatly diminish the value. Custom updates are possible, but at the buyers discretion and caution. I love bargains. I say go for it. My Bozak speakers I gave away to a friend are selling for real money 40 years later!

I had a local speaker repairman restore my bottom foam surround woofers of my Legacy Signature IIIs with rubber surrounds.  Strange that the other two woofers and my Focuses all used rubber surrounds in great shape after 30 years.

rubber surrounds will last forever, but they don't sound as good as foam surround, which have limited life span, especially if you live in hi humidity area