Do Your Research
For someone that doesn't know much about cars it's easy to confuse one part with another. Know exactly what part it is that you need and research how much it typically costs. If possible, bring the car part with you to find an exact match.
Scope out the Providers
There are numerous options when purchasing used auto parts. You can buy directly from a junkyard, have your mechanic find used parts for you, or purchase the parts online. Ask around and get recommendations. Check out the businesses online to see if you find any scams or negative feedback. You don't want to buy used auto parts from a shady company.
Ask about Return / Exchange Policies
There is always a risk factor when buying anything used. You don't want to buy a part only to have it not work properly. It's important to feel comfortable knowing that there is a clear return / exchange policy and some kind of warranty. Have a firm understanding of the warrantee before making a purchase. You don't want to get scammed because you didn't read the fine print. If they don't even offer a warranty, find a different place that will.
Get the Part History
How old is the part? How many miles the previous car go? This information can help guide your decision.
While numerous used car parts are just fine, think twice about parts that directly correlate with safety, especially if you are often driving around passengers and small children. Some parts are best to purchase new.
Think About Color Match
This mostly pertains to body parts that are seen on the outside. While a part might fit perfectly, it may not be the right color and it may be difficult, if not impossible, to paint or stain it so that it matches. While some may not be worried about this, for others this would be a deal breaker.
You Don't Have to Go to the Big Guy
Sure there are plenty of well known national chains that sell used auto parts, but they are likely more expensive and the parts have the same quality as those that you would find at a neighborhood yard or a smaller retailer online. Do your research and compare prices. Some yards may even be willing to negotiate a price with you.