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?
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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.
I bought my Grado Reference cartridge from an Associate Professor at Michigan State Univ., and have been very pleased with it. His advertisement here on A-gon said the cartridge had about 100 hours, and from what I can tell that was accurate. I would not normally buy a used cartridge, but the MSRP on a Grado Reference was more than I could justify, so buying it used seemed like a good way to upgrade. It turned out to be an excellent buy. I recommend, however, that the prospective buyer contact the seller by phone and discuss the cartridge and its condition in some detail.
I second Sd's last suggestion very strongly. Other than the cartridge being 3rd hand (the seller would clarify) or the way it's been used, the way it's been mounted -- vta, azimuth, anti-skating, etc -- can age it prematurely... A friendly, in depth conversation with the seller is a must and an occasion to make new acquaintances too!

Maybe I have been lucky, but I have never gotten a bad cartridge used, either here or even at ebay. I have purchased a half dozen used cartridges, and none were ever misrepresented to me by the seller.
The last thing I would want to do is to sell someone a cartridge with a worn stylus , and allow them to ruin precious lp's, in order to make a 100 or 200 bucks. I think most audiophiles are good people, and feel the same way,.
I've only heard of poor experiences buying Benz cartridges (supposedly NEW) from a Hong Kong dealer in Hong Kong. I can't imagine that regulars would dare pass off trash here.