Definitely be sure you buy a used camera from a reputable source. A couple of good places to buy used are:
These stores carefully inspect and refurbish all their used gear before selling it, and if it does turn out to be a lemon, they have very good return policies and warranties. I would buy from either of these places in a heartbeat if I needed any sort of camera gear. They also have fairly competitive pricing. Obviously you’ll probably be able to find one cheaper on ebay, but you never know what you’re getting from ebay.