While it may be economical and ecological to buy lightly used items online or from thrift shops, there is one item a parent should never buy used: car seats. The safety regulations for car seats are constantly changing, as crash test results reveal new ways to improve them. The designs for car seats change so frequently that they are often stamped with an expiration date, lasting only about six years after purchase. Since car seats are designed to withstand only one crash, there also is no way of telling if the seat is still usable or not. For the safety of your child, buy a new seat.