I've had an experience but only avoiding mail orders -- only local pickups. I've bought Benz Glider and I checked the diamond head and cantilever with pocket microscope. The cartridge appeared to be in superb condition and it sounded as it was new having 450hrs+ on it. I guess that diamond stylus isn't getting affected by even 5000hrs of using -- only the cantilever from getting on and off the record + loading current that flows throug it. If the cartridge is well maintained, it could last for 5000 hrs+ having no problem. If you'll ever see that there is black dust on the magnet instead of gray or white -- that is the time to retip it or change it, also that means that the cartridge had it's vast time used even if well maintained.
Buying used Cartriges
I quick gander at the classified section here on A-Gon will reveal that cartridges sell very well. How do you all protect your selves in buy something that you cannot verify the age and condition of? The sales are not being made with long term, high feedback members. Many are zero feedback users. Most sellers state, used "50-100 hours", with models that have been on the market for several years. I have made 3 cartridge purchases here on Agon and have been lucky so far, but am in the majority or minority?