You need an inverted microscope with at least a 40X or higher power objective in order to see the stylus shape and wear. The diamond should never be "sharp;" that would permanently damage a record. The profile needs to be smooth and polished, otherwise, if it has sharp facets due to damage, it will act as a lathe and will scour and scrape off the undulations in the vinyl.
Personally, I never buy used cartridges. Too much risk. It's not worth it.
Personally, I never buy used cartridges. Too much risk. It's not worth it.