...I have to ask that you take a moment and read my friend, Anne's, recent experience.
Anne and her family's story at Car Seat Nanny
For all intents and purposes, the crash Anne's family was in could have been a multiple fatal. My husband has covered many crashes just like this, with much different results.
Please, please, please. Never buy used car seats. Have your car seats installed by a law enforcement officer or car seat installing specialist (like Anne herself). Use them. Every single time you are in the car with your children. Be an example and protect your children's parents. Wear your seatbelts. Every single time.