It's not advisable to buy a used seat that is more than a couple of years old, simply because of wear and tear and having no guarantee as to the structural integrity of the seat. However, a used seat is better than no seat. If you do find one that is in good condition that was originally compliant with regulations, always get it checked by the manufacturer, baby store, or through the NHTSA directory where you can find a car seat inspection station. You can also search for a certified technician through the National Child Passenger Safety Certification Program to give it a once-over.