When do carseats expire?

On the seat, and in the manual as well there will be a Date of Manufacture. Most seats expire 6 years from this DOM, other brands are 7, and in some cases even 9. Six years however, is the most common, and is what is likely for all of your seats. Some one on this board has a "Used car seat check list" that hopefully they will post (I really should steal it :P) that will let you know if they are ok to re-sell. If they are, I would make sure you give the new owners the manual(s). If you lost them, most companies will send you a new one for free, or just a small fee which you would recover when you sold the seat.