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

I wouldn’t pay 25% over MSRP for anything.

^ You pay for what you get!

Take it to a specialist with a good microscope.  The specialist can tell you the condition of the stylus, the suspension and the magnet coil assembly if it is a "naked" body cartridge.  Retipping and rebuilding cartridges are expensive services.  Caveat emptor.  The cartridge could have been dropped, dragged across a record, bounced off the turntable or misaligned in the tonearm.

If a Cartridge rebuild service chipped in with their discoveries made on a Cartridge that an owner thought was in quite good condition, and was surprised at the assessment offered as part of the Inspection.

I am sure there will be few descriptions of a discovery made that will be hard to accept as being for real.

The 'fragility' of the Cartridge is the concern, and is possibly the main reason that requires a special attention to ensure the overall function is being considered when purchasing one either new or used.

A new cartridge can not have have come through the stages of delivery in the finest of condition. When received it could have developed a fault. A Cart' can also have slipped through a usually rigorous QA check prior to being packaged. 

Combine the above with usage to what ever degree, and the variables can increase substantially for a Cart's function not being as per the designed for operation.

There is absolutely no harm in treading carefully and finding the method that can be most satisfactory to the buyer, for proving their monies are being spent quite sensibly. 

Individuals who spend less on a used vehicle than some would spend on a used Cart' would not think it strange to consider or action the vehicle to undergo a third party vetting.