Generally the "shelf life" of a car seat is 6 years.  One of the main pieces of reasonig for that is the everyday wear and tear a seat goes through is enough to wear it out and cause it to not work as intended.  The straps begin to wear run the risk of breaking in an acident.  The other thing with an inappropriate seat is if you get pulled over and your child is in one, at least here, in one the officer can confiscate it right then, or write a fix it ticket.

Getting a seat second hand is usually not a good idea unless you know the history of the seat.  Any that have been in an accident should be disposed of (usually the insurance company will cover that expense.)