For moving magnet carridges (AT150MLX, 2M Black, Nagaoka MP-50/500):
The cartridge itself rarely fails. They're usually good for decades of use. What matters is the condition of the stylus. If you get the body cheap enough then you can go ahead and purchase a new stylus for it. If you're purchasing with the intent of using the included stylus then purchase from a trusted seller who can state the approximate number of hours on the stylus, or who can examine the stylus under a microscope to determine its condition. Otherwise it's a crap shoot at best.
The cartridge itself rarely fails. They're usually good for decades of use. What matters is the condition of the stylus. If you get the body cheap enough then you can go ahead and purchase a new stylus for it. If you're purchasing with the intent of using the included stylus then purchase from a trusted seller who can state the approximate number of hours on the stylus, or who can examine the stylus under a microscope to determine its condition. Otherwise it's a crap shoot at best.