Experience purchasing a used high-end cartridge

Unless you're very lucky all in all you won't get more value than you pay for. I started in this hobby trying to get some great carts used and had limited success. Then moving up the line got hurt on a couple. One in particular- Airtight Supreme, the suspensions are the weak point and once worn out you're done. 

I would only buy from the manufacturer who has certified it as good as new, or from a dealer I knew very well, at a store I can visit so they can do the set up. I was lucky to get a VDH MC One Special  from a dealer I knew for 20+ years who had a customer trade it back to him shortly after he bought it for a VDH Frog. He doesn't try to make money on the trade-ins so I was able to get a tremendous deal (over 60% off list). The stylus lasted 9+ years with a very good amount of playing time. VSH styli last longer than most due to the design, created by AJ van den Hut himself. 

that's van den Hul (damn spellcheck)

I’ve purchased a couple used cartridges. A Grado ReF Platinum was only $250. But I did acquire a Benz Ace SL for $650. The cartridge was sold by a dealer with a guarantee so truly no risk. I’d owned the cartridge some 12 years earlier and had been desiring another one. The cartridge is an exceptional moving coil, and impossible to find used anymore, which is why I paid more than I should have, but you can’t take it with you. 

I have sold several used cartridges over the years. I have also purchased several used cartridges. I agree that there is risk involved but not really any different than speakers of amplifiers. When a cartridge goes bad, there are a very limited number of things that can cause it and almost all of them are repairable.

When I sell, I will take many pictures to prove the authenticity and normal function  as best I can.  I also take USB microscope pictures of the stylus and cantilever. I like this information when I buy as well.  

I will take a cart back, no questions asked if the seller isn't satisfied.  After all, I know that the cartridge works great and why not keep it a little longer?