Using a car seat isn't optional, and neither is making sure the car seat that you are using is in good working order. I have been researching car seats in the past few weeks and have made some major changes in the way that my kids ride in their seats.
For example, we didn't, and don't plan to, turn our one year old forward facing until we absolutely have to. There are some great articles online about the importance of extended rear facing, such as this: http://www.car-safety.org/rearface.html
There are also many, many reasons that you don't want to pick up a car seat that has been used. I *love* recycling and thrift shopping but I will never buy a second-hand car seat since any seat that has been in an accident, even a minor accident, should be trashed.
Car seats do have an expiration date and should be destroyed after the date on the side or bottom of the seat. Most car seats have a six year expiration date, while others may have a ten year expiration. Don't hesitate to toss a seat that is expired according to the label. The date that you actually purchase the seat may be much shorter than the six year time span.
You also need to check for current recalls of car seats. You can check the manufacturers website, or you can check here to find a list of manufacturer's websites. The name of the company that made your car seat should be written on the label or directly on the car seat.
Finally, some recommendations for car seats. Personally, we love the Recaro brand but Britax is also an excellent brand. Don't worry if you can't afford these brands, however. Using a car seat from Wal-Mart or Target is fine as long as you follow the guidelines for use and check the expiration dates.