Steer clear of used car seats if you cannot get an accurate history of the car seat's use. You must know a car seat's crash history to determine safety. Buy used car seats only from people you know well enough to trust their account of the car seat's crash history. Car crashes can cause hidden damage to car seats, and the car seat could fail in a second crash, putting your child at risk of injury. Check to be sure no alterations have been made to used car seats. Choosing the Best Baby Car Seat - Before You Buy a Baby Car Seat - Car Seats Buying Guide
Use caution when buying used child safety seats. Buy new, or from someone trusted who knows the child safety seat. Make sure any child safety seat you buy has:
instructions
all necessary hardware, straps, buckles, harnesses and chest clip
not been in a collision
not been recalled (check with the manufacturer or Transport Canada - 1-800-333-0371)
not expired or is not older than 10 years if no expiry date is given
no discoloured (stress) marks or cracks and the harness is not worn or torn