YSK that infant car seats expire, generally 6 years from date of manufacture.
Why YSK: Materials degrade over time and technology/regulations for safety get regularly updated, so an old car seat may not actually be safe.
While baby stuff IS expensive and it's great to get most things secondhand, pay close attention to the expiry date of car seats and make sure you trust that the seller is telling the truth when they say they've never gotten into any accident or collision.