First of all, let us tell you what you should not do with them. You should not continue using them or giving them for other people to use. Why? You might consider reading this article of ours. In short, materials car seats are made of are capable of fatiguing. If used for a long period of time, they can develop cracks and accumulate structural damage. That makes using them past their expiration date pointless and dangerous.
Unless you plan on using the car seat as a source of materials either for creative activity or something similar, you have to dispose of it properly. Under no circumstances should you sell an old car seat or even give it away for free, doubly so if the car seat in question has been in a crash situation. In fact, no car seat should be used by anybody including current owners after a crash, even if it has been produced only a day before that and appears undamaged.
The best way to dispose of an old car seat is to give it away for recycling. Check out this site for any tips regarding the proper procedure. Alternatively, you can dispose of it in the usual way, however, you must take steps necessary to render the seat unusable and unsalvageable. Break it down to pieces. We recommend that you use power tools to do that. Remember that it is your responsibility to prevent someone from using your child’s old car seat. Many people would readily pick one up not realizing how dangerous it would be for their little ones. Forget about “one man’s trash is another man’s treasure”, this proverb is completely inapplicable in this instance.
Passing an old seat down is only possible if the seat is yet to expire. Even then, it is ill-advised.