Here are some general guidelines to help you buy used auto parts in any situation
Know what you need. Be sure you know exactly what you want to buy before you shop. If possible, have the old part with you for comparison. Because used parts are often not returnable, you don't want to waste money on the wrong part.
Ask questions. Don't worry about looking like a novice while you stand at the used auto parts counter. They deal with amateur mechanics all the time, and most guys will give you credit for fixing it yourself even if you don't look the part.
Be sure the part is returnable. Sometimes you'll end up with the wrong part no matter how careful you plan. Check to see that where you purchased the part from will exchange it or issue a refund.
Decide whether used is good for this repair. If you crack a tail light or break a motor mount, a used part is the perfect low-cost solution. Parts that wear out anyway like tires, pads, and brake rotors should probably be purchased new.