What is it with all the wet necks and used cartridges!
Use your head and common sense when shopping and you will be fine. Remember that one, common sense, that's right the least common of all senses nowadays it seems!
I have bought and sold many used cartridges here and on eBay.
There still are a LOT of very honest people in this hobby.
How else is one to try and acquire and use some of the better vintage mm carts apart from taking the plunge and buying used?
If you do not want to take a chance personally that's fine but do NOT denigrate the process as a whole!