As long as the buyer knows what the risk factors are for gray market and is buying from a reputable seller, it presents a decision. I know that Koetsu supposedly will not work on any gray market cartridge even out of warranty, or so I'm told. A lot of people go the third party retip route, which raises a different set of issues. My Airtight Supreme, which had a lot of miles, got repaired by Peter L.- he did not change the factory cantilever but instead inserted a new stylus into the original cantilever and realigned the motor assembly. That was money well spent. On the Koetsus, I'd send them back to the factory for full refurbishment, rather than have a third party work on them (though there was one vendor in France who was known for its impeccable work on them).
It's all trade offs, cost and risk. You, the buyer, have to be the judge. High end cartridges are now crazy money.