Have a good look at the manual (there is a good cache of these online) and go through each section ensuring your seat is as good as new and being used as per the manufacturers recommendations.
There are good resources out there to help you with any queries you might have!
Remember car seats are for the safety of our children, it is ALWAYS better to be safe than sorry. What's a few hundred dollars if it helps save a life?
Important- If car seats were installed in a car that was involved in crash, you need to replace them, even if they pass the steps above. There may be micro cracks in the plastic or metal parts of the seat that are not visible to the eye. Your insurance should cover replacement of the seats.
We don't recommend buying or selling second hand car seats, even if they pass the test above.