Experience purchasing a used high-end cartridge

trust but verify with an independent expert.

In other words, distrust until verification. If you find a seller willing to let you ship his cartridge around and have it handled by some third party, sure go for it without a second thought.

If you are inferring that there is some risk to the seller, I would think not, if you both (seller and buyer) choose an inspector who is well known to the audio community and with an impeccable reputation, like Peter Ledermann, etc.  Or even better, perhaps the seller has particular experience with one professional retipper that he or she would prefer to use.  Possibly also Wally Tools does pre-purchase inspections of this kind. It's well worth the extra cost to the buyer, if we are talking about a cartridge that costs thousands of dollars.

Lots of ifs in this hypothetical. Sure, “if” all are satisfied, go for it.

I am saying that inspection by a competent disinterested professional third party is what I would require if I were to buy a pre-owned megabuck cartridge. Speaking for myself only. I guess you don't agree, and that is fine.

No, I agree that I would need your criteria met if I were to buy this hypothetical used cartridge. How many times can I say go for it? I don’t buy used cartridges so the point is moot, but I’m submitting to your hypothetical. But I also think this transaction is unlikely to occur in reality. Most sellers like a glide path, not additional complication and risk. Additional shipping is a risk. And how many sellers do you think would say, sure, here’s my cartridge, have an expert vouch for it, however long that takes, while I don’t sell it to anyone else. And Peter can take months to do work. Could happen. Sure why not. Once again, go for it.