Perhaps I have been lucky, but I have bought four out of the six cartridges I've owned, since getting back into vinyl 11 years ago, used, and they have all been in good shape. I've bought from one person on Ebay, one from a guy on Audiogon and the other two from two friends. (Obviously the later two were obtained in a more reassuring manner then the first two.). I think the best thing you can do is to call the seller to discuss the cartridge, their system, and other pertinent matters as a way of feeling them out. You need to make sure you are comfortable before going through with the sale.
My two cents worth.
My two cents worth.