Verdict: Keep it or donate it, unless it's been seriously compromised.
Why? Unless it's been recalled, your car seat (my favorite is the Peg Perego Viaggio) should last through a couple of kids. The exceptions: car seats that have been in an accident or damaged in some way. Those you'll need to toss out. Be sure to give your old car seat a thorough cleaning between babies, though. Spit up, spilled milk, and dropped snacks can make for some nasty seams!