Used, Crashed and Expired Seats
Car seats are designed to protect your child in the event of a car accident, and an accident puts the seat under a huge amount of strain. This can cause damage to the seat, which stops it from being effective in a subsequent crash. Even if there is no visible damage, you cannot be sure that the seat is still safe, and you should therefore throw it away. Cut the straps before you dispose of the seat, so that nobody else can use it.
While it is not illegal to use a secondhand car seat, you are advised against it. Unless you are given a used seat by a friend or relative and you know it hasn't been involved in an accident, you can never be totally sure of its history.
In some countries car seats have expiry dates after which they should not be used. These can be anywhere from five to ten years after the date of manufacture. Here in the UK car seats do not expire. All seats that conform to Standard ECE R44/03 or R44/04 are legal to use. R44/03 came out in 1995, so you can legally use car seats for a very long time. But there are a few reasons why it's not a good idea to keep using a seat that's more than about ten years old.
They are made of very strong materials that will last for years. But the older the seat, the harder it is to be certain of its history. And even if you bought it new for your first baby, by the time your third or fourth child is born you may have lost the instruction manual and not remember exactly how to install it. And safety standards are updated and improved all the time, so an older seat won't have the latest safest features.