Used equipment is a great way to save some cash, but can also be a risky investment. If you plan on buying used equipment, make sure that you know what to look for.
Stay away from buying used rope, harnesses, or slings. This is because they are susceptible to deterioration with no visible clues. For example, if the rope has been exposed to battery acid fumes, it is radically reduced in strength but there are no visible signs.
Hardware, however, can usually be bought used without any problems. Cams, stoppers, and carabineers can last a lifetime as long as they are taken care of properly. Just make sure that they move well and don’t show any major deformation. Carabineers should be inspected for tiny stress fractures around the pins in the gate. With climbing hardware, if it looks good, then it usually is.
In rock climbing, it is very important that you trust your equipment. Bottom line, if you don’t trust anything that you haven’t bought new yourself, then don’t use anything that you haven’t bought new yourself.