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

I think enough people have answered to confuse you on buying used or new. I want to share, instead, how I keep track of my cartridge hours. I once purchased a box of hand tally counters (the sort that you see where places are trying to count how many people have entered.) Maybe 12 bucks for a pack of 6 on Amazon when I acquired them. 

 With a label machine, I indicate the cartridge and the date I acquired it. Then, every time I play an album side on that cartridge, I click the appropriate tally counter once. This may sound onerous, but between using a carbon fiber brush, some blue tak on the stylus, and pressing the tally counter probably requires 20 seconds maximum. More like 10. It’s a worthwhile routine to develop and keeps needle and vinyl in excellent condition. (This assumes you first cleaned your vinyl properly on acquisition). It’s just like brushing your teeth—proper habits preserve health and money. 
I then divide by three to give an estimate of the hours, knowing that this will yield a slightly higher number since few albums sides are a full 20 minutes.

Then you are empowered to decide how many hours a given stylus should last, or when you need to scope the stylus. Simple.