Cartridge without serial?

Cartridges have serial numbers? Huh. Did not know that. Oh, and you can't warranty anything without a serial number. Who'd a thunk? Learn something new every day.

There are a few like juki on Ebay and Kron here on Agon that sell these "gray market" cartridges. There are buyers (from the number of reviews) that would rather pay these discounted price than the full retail price (plus tax) that a dealer would otherwise charge. What is your risk level?  If something happens to cartridge during the warranty period, you will not be able to send it to the dealer, but rather to repair places like Soundsmith or VAS Audio at your own cost - the wait can be long. That's the trade-off. If its the two sellers that I mentioned above, I wouldn't be too worried about the authenticity/quality.  

Actually a serial number itself if not a warranty, you can get warranty when you’re buying from official dealer and it’s dealer ship your cartridge to the manufacturer (not you).

Grey market seller often erase serial numbers or replace them with fake serial numbers, so the manufacturer can’t track the source.

I have serial numbers on every cartridge I got in my big collection of cartridges, except for one ZYX Airy III cartridge purchased from 2juki from Hong Kong! Upon my request the manufacturer confirmed it was a fake serial number, so no warranty!

As been stated million times on this forum:
Refurbishing some special high-end cartridges using different parts from re-tippers is a bad idea! 

Manufacturers of ultra high-end cartridges never refurbish or re-tip them, with warranty you have trade-in program to ship your old cartridge back to get brand new cartridge (even next model) with special discount.