I am completely in line with Ekobesky's comments. Over the alst 4 years, I have sold a Kontra A with about 100 hours, a Benz Ace with about 50 hours and ZYX R50 with about 20-30. In each case, I really wanted to try for example a well known combination that was recommended to me (Ace on VPI Scout) or just know the flavor of the ZYX cartridge line.
I would trust a person for a used cartridge as well if the three things are fulfilled:
(a) positive feedback, (b) length of time on A'gon and (c) their experience with analog
Also take a look at the forums here and on Asylum - several people are active in both places and I would trust most of the active members more than someone how just comes here to the classifieds section to turn over goods.