Are 500 hours too many for a used hi-end cartridge?

Your responses have been very helpful. I think that I will avoid high hours used cartridges. Too many unknowns.
I’m now thinking that about $1000 on something like a new Hana ML or AT ART9 is a better way to go. While possibly not as fine as some $2000 cartridges, at least I’ll be using the cartridge in its prime. And these have great reviews.

My approach, if I were to be tempted to buy an expensive used cartridge where the seller estimates as much as 500 hours of use, would be to suggest to the seller that the cartridge should be evaluated for stylus wear, etc, by a competent third party professional, like SoundSmith for example.  SS provides such a service for a fee.  You could then work out with the seller who should pay the fee for evaluation.  For example, seller pays if SS says the cartridge needs a rebuild; buyer pays at least a fraction of the cost, if SS (or other) says the cartridge or stylus has at least half of its life remaining (or some other similar fraction of its useful life).  The buyer would probably have to state in advance what amount of wear is "acceptable". There are other details that could be worked out between two reasonable sincere parties.  (I am not a lawyer.)

There is no direct correlation between price and quality when it comes to $1000 or $2000 cartridges. More important is perfect tonearm/cartridge matching, capability of your phono preamp if you're using LOMC and your personal satisfaction with one or another model, they can be low compliance or mid compliance, silver soil or copper soil, different stylus profile and overall different sound presentation (different flavor).  

Before you will buy any LOMC ask your dealer about service, because when the stylus will be worn you will have to replace it with equal diamond, you'd be better if the manufacturer can replace old cartridge with new one for special price (many manufacturers can do that), they have special program for upgrade, customer support via their official dealers only. 

Art-9 from your wish list is probably best bang for the buck when you're looking for high resolution NEW low output MC with decent stylus profile. You can also look for Dynavector and Shelter cartridges. 

Current analog front ends are:
Hana EL, Pro-ject RPM-9 with Evolution graphite arm. EAR 834P phono pre.
Vintage HOMC Adcom Crosscoil with Soundsmith ruby upgrade (really), Oracle Delphi V with granite plinth, SME 309 arm, Manley Chinook phono pre.
Both systems feed vacuum tube amps.
Previous favorite cartridges included Ortofon Cadenza Blue, Sumiko Blackbird.

Look at it this way. The vast majority of the sound of a cartridge comes from everything other than the stylus. The vast majority of wear on a cartridge is only the stylus. Therefore the one thing most likely to be worn out is also the one thing least likely to matter. 

As usual when stating something perfectly obviously true people will scream and argue this cannot be. Yet it explains so well the experience of so many who have used and enjoyed many really high end cartridges for years and years and thousands of hours. 

But then I am a genuine example of what someone around here called an iconoclast. I just don't see how hours matter. What are you gonna do? Scope it every hundred hours? And then what? Oh, its worn. Guess I better replace it. Even though it still sounds better than when it was new. I don't think so. What you are gonna do is use it until you feel like getting something better. 

You are never even once gonna look at the stylus. That's what I think.