You should not use a car seat that has been involved in an accident. The seat could be structurally damaged and no longer provide proper protection. Avoid using second-hand seats. They could have been involved in an accident or have missing parts which render them ineffective at protecting your child. Be consistent in the use of the car seat.
Some parents use car seats only for long trips.
Actually, most car accidents occur within 25 miles of home. Since you never know when an accident will happen, use your car seat correctly every time. While you are traveling, do not remove a child from a car seat or allow seatbelts to be unbuckled. If your child needs to get out of the car seat or the seatbelt for any reason, wait until you can safely pull off the road. Holding a child on your lap will not provide protection even in a minor accident.