I have been fortunate buying used cartridges, and I have done so. But the way I look at it I want to buy a cartridge that is above a certain level where it makes the idea of retipping an economic possibility. My opinion is this. Cartridge manufacturers buy their diamond and cantilevers from suppliers such as Namiki and Ogura. If a retipper uses diamonds from them, such as Namiki, if you get there higher quality offerings the differences are going to be minimal. I once had two Koetsu Urushi and I lost the diamond on one of them during playback. I had it retipped by Andy Kim with the appropriate Namiki diamond, I compared it to the OEM cartridge I had on hand, and I could detect no meaningful differences.
I bought a Transfiguration Audio Proteus from a dealer he took in on trade in. Price was great $2K for a $6K cartridge that was vetted with a reported 500 hours. I had it sent straight to Andy and he evaluated the tip and reported minimal wear, with an estimate of 300 hour range use. But that is only an estimate, as he said minimal wear viewed under a scope. So I use up the rest of the hours, which could be another 1700 and I send it off to someone like VAs, or Expert Stylus and put $500 into it for a tune up and new diamond. I think this makes good economic sense. I would not be retipping cartridges like an Audio Technica OC9 III. Just not the value there.
If buying from a private party I tend to look at his equipment, how long he/she has been doing this, and what boards they have participated in. Experienced owners tend to treat their cartridges well. There is one fellow who sells on AG and USAM who is sort of a dealer really who I would not touch his stuff with a 10 foot pole. Everything has 100 to 200 hours, and he has always got his stuff from famous "collectors". He has interesting cartridges come though, and I am tempted, but if you read enough posts about him there are people who are quite unhappy. Not worth rolling the dice with his products in my opinion.
Buying used cartridges is like buying used cars. Always a reward versus risk calculation. I have been fortunate doing it, but I have had a dud once in my early days of trying. Fellow got a bad cartridge by me, and I got it repaired by Andy but it died an early death and was not repairable after that. I got my moneys worth out of it, or so I tell myself.