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

I'm down with no pre-owned moving parts. That applies for speakers too but careful audition/inspection can override it for them.

I’ve just never done it. Won’t either.

One method to avoid upset is to arrange an agreement between Vendor, Buyer and a Third Party, who is to receive the Cartridge, the Third Party will need to be recognised for being able to assess the condition of the Cartridge and the assessment will be the agreed guidance for the establishing the condition.

If all appears to be as described in the Ad, then the Cart' can be paid for as agreed.

If the Cart' does not meet the adverts description on condition, the Cart' can be returned to the Vendor, or an alternative arrangement might be agreed on the outcome of the assessment.

This obviously incurs an extra cost, for the assessment and the extra shipping.

If I were to be keeping the Cart' as a result of the inspection/assessment, I would request a through cleaning of the assembly, and this will usually be a charge that has the inspection offset against the overall cost.   

I have done this in the past with a Cart' and the outcome was fine, this Cart' eventually become a donor for a rebuild as the Cart' was without any concerning issues when re-inspected.

On another occasion I sent many of the supplied images of the Cart' to a respected Third Party Service and was given a reassurance that their was nothing noticeably concerning from the photo evidence. I accepted the evaluation and own this Cartridge as a result.

I always do this arrangement when buying vintage valves, the Vendor agrees to my having the Valves re-tested.

This has proved to be very prudent, as a few low measurements have been discovered by the Third Party Service that I use. In this case the service is only a short drive away from my home, and is a fellow HiFi enthusiast.

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:

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.

I don’t have a problem buying used as long as you buy the seller.

also if local... that’s a plus.

50%.

Through detailed conversations within the past five years, the way I have come to understand the initial concern about the condition of the Cartridge is to investigate the Bobbin Assembly.

This is quite different to my old thoughts and methods to be only concerned about the Stylus condition, Mounting! > Wear ! 

In a Layman description, the Cantilever Anchors to the Bobbin Assembly and sets up the mechanical operation that will enable the Stylus to perform as per the designed for operation.

Any compromised materials discovered at this place in the assembly will be detrimental to the overall performance.

Hence, my skill set does not reach to this level, so I farm the inspection out to a much more reliable assessment, the bonus is the Stylus will also be evaluated along with other materials within the assembly.

When it is viewed with this in mind a Third Party Inspection starts to show its real value to the buyer.