Are 500 hours too many for a used hi-end cartridge?

I have been looking for good used mc cartridges on Audiogon in the $800-1000 price range. Most cartridges at this price advertise 20 to 200 hours. A few questions:
1. Are the advertised hours believable, since turntables do not have elapsed time meters?
2. Is cartridge age more important than playing time?
3. Is 500 hrs too high for the purchase of a used mc cartridge?