Plenty of good advice here.
It's always a bigger gamble buying used but there are a few tips I've gathered along the way.
As my brother often said it a good idea to look at the seller of a used car as much as the car itself and the same rule can be generalized here.
The more information you are presented with the better. It's amazing how folks selling used phones and tablets etc still somehow forget to mention that the rechargeable battery is now nearer to 50% than 100%.
Like everyone else I'd usually want a much bigger markdown for when buying something with plenty moving parts where wear and tear is more critical.
Even a well looked after Lexus or Toyota will fetch a lot less if its mileage is high. (Yes, I've been watching too many Scotty Kilmer videos).
So I'd never consider paying more than 50% for any used cartridge.
As said earlier there's always going to be some risk involved, so yes buyer beware. Every seller is aware that no matter how honest they try to be there's no real way of knowing when some part is about to fail.
I've never bought a used a used cartridge, nor sold one for that matter, but if I did I'd do well to remember the advice given above by @mulveling
It's worth repeating again as it's not just the tips that wear, the suspension can also wear or get damaged by one too many careless needledrops.
You want to see pics that show at least two things:
- A straight cantilever that is not askew, and if possible an "on record" pic to show it’s not riding too low.
- That the previous owner(s) have practiced good cleaning hygiene. Particle "beards", a blackened stylus, or crust atop the cantilever are bad signs.