Would you pay more than 25% of MSRP for a used cartridge?

I seriously doubt that anyone would sell you a high-end cartridge (> $3k) "low hour" cartridge for 25% of MSRP - highly unlikely.  Maybe a Denon 103, but nothing more.  Buying used cartridges is risky, but people do buy used cartridges. The popular models get snatched up quickly like the A95, XV-1S etc,. If the seller had excellent feedback and it was a model I wanted, I'd pay more than 50 % for a "low-hour" (< 200 hrs) cartridge.

lemme guess. you low-balled a seller at 25% of retail for a scarce and hard to get cartridge that rarely shows up new let alone used. you were then turned down multiple times by many sellers. your rationale is that all sellers are liars and not to be trusted re claimed hours of use, etc. got it. 

I guess as audiophiles, we’re all gonna be OCD about SOME things - but hopefully not all things. I’ve been pretty relaxed about buying high-end used cartridges. If the seller has good feedback here, I’ll trust them - especially if they look like a hobbyist who knows what they’re doing. It’s generally worked out pretty well for me. You want to see pics that show at least two things:

1. A straight cantilever that is not askew, and if possible an "on record" pic to show it’s not riding too low.
2. That the previous owner(s) have practiced good cleaning hygiene. Particle "beards", a blackened stylus, or crust atop the cantilever are bad signs.

You could also be paranoid about L/R channel balance, but the only cartridge I’ve encountered yet with an imbalance of >= 1dB was 30 years old.

The last cartridge I bought used was an Ortofon A90 from here - and I forget the actual price but I guess ~40% of MSRP? It's a real gem; if anything the seller undersold it. 

I just bought a used cart here. Ortofon Cadenza Bronze. Great deal. About 40% off list give or take, these carts never go on sale. It’s a good way to try different sounds. As with anything mechanical, buy the seller, not the item. In this case, the cart was a dealer’s personal cart. Listening to it right now. Excellent!

i have purchased many used cartridges

it is all about doing homework on the seller, knowing when to back off/move on... and buying the potentially risky ones in a way that allows proper recourse in the worst case scenario