It is best to avoid relying on a used car seat.  If you must buy a used one, be sure to check the following:

• Most  recommend against using a car seat that is more than five years old.  You should check with the manufacture to find out when the company recommends getting a new seat.
• Make sure the seat was never in a crash.  Child car seats are built to withstand one crash only
• Make sure you know the manufacturer name, model number, and manufacturing date before you buy.  The seat should have a label attached with this information. With this label of information, you can check on whether the product has been recalled
• Make sure the seat show no signs of wear and tear.  Avoid seats with cracks, dents, tears, or missing parts.

There are several places you can obtain a new car seat for a low price or even free check with your local Fire station some sell them for around $30.00 also check with you local Heath Dept. they are usually able to give you one or point you in the right direction.  Also if you need help installing your car seat or are not sure if you have installed correctly the local Fire station will usually check the car seat for you.