Experience purchasing a used high-end cartridge

I have purchased many MC carts used through the years. 
I have never been disappointed. I once purchased an AT-ART 9xA for $650

Seller put clock time on every cart. His feedback was tremendous. I was very satisfied. I have purchase a few more carts. 
Huge upgrade. 

I have used the Condition Check request for Valve Purchases.

I negotiate as part of the sale, the Valves are to be tested when I receive them, and are then kept on returned dependent on the measurements taken.

If the Vendors measurements are close to ones taken on my behalf, I have honoured the Purchase, in some cases the measurements on my behalf have been much improved over the Vendors recorded values.  

Having a assessment agreed as part if a sale agreement is one more step towards Caveat Emptor. 

I have bought at least 8 used cartridges.  I only buy where the seller specifies his usage and I discount that heavily.  So if somebody says < 20 hours I assume it has at least 50 on it.  I would almost never buy a cartridge where a seller claimed more than 100 hours unless it was very deeply discounted.   I did make an exception for a Koetsu Urushi Gold I recently bought where the seller claimed 250 hours but the price was good and it is impossible to buy new Koetsu's anymore.  My only disappointment was a Soundsmith Zephyr MKII and I really have no idea if the seller misrepresented it or it just is a bad cartridge.  

Unless you have a lot of money the only realistic way to try a lot of cartridges is to buy them used.  Lots of vinyl junkies have multiple cartridges which does mean there are many genuinely low hour cartridges that might come up for sale.

I would never buy a cartridge that was retipped or where the user claimed "only 500 hours and it still sounds great".  Diamonds aren't forever when it comes to cartridges.

Personally I don't do it. But if I was, I'd look for dated proof of purchase and I would only buy a relatively recent cartridge whose supposed hours tallied with its age.

ive done it. person-to-person sale. i vetted the guy beforehand, and it was a low-risk situation.seller gave me return policy. didnt have to use it.