That safety seat you scored at a garage sale for a fraction of its original price may seem like a bargain, but it could cost your child his life. The same goes for that older-model seat your sister gave you after her child outgrew it.
Not only are used seats unlikely to come with the manufacturer's instructions (vital for correct installation), but they could be missing important parts, have been involved in an accident (even unseen damage can affect the seat's functioning), fall short of current safety standards, or have been recalled due to faulty design. Moreover, plastic gets brittle as it gets older, so a seat that's too old could break in a crash.
If you must use a secondhand seat, make sure it has the original instructions (or contact the manufacturer for a replacement copy), has all its parts (check the manual), has never been involved in a serious accident, and hasn't been recalled. (Check your seat's recall status here.)
In addition, to avoid the dangers of aging plastic, SafetyBeltSafe U.S.A. recommends sticking with car seats that are ideally less than five years old and definitely less than ten years old. You can usually find an expiration date stamped somewhere on the seat.
More can be found on this at BabyCenter.com.