The core risk with a secondhand car seat is invisible crash damage: a seat that’s been in even a minor collision can have compromised structural integrity with no visible signs. If you don’t know the seat’s full history from new, that’s a real risk, not just caution for its own sake.
Car seats also have an expiry date, usually 6–10 years from manufacture, printed or moulded somewhere on the shell. Always check this before considering a secondhand seat, and also check for a manufacturer recall using the model number.
If you do go secondhand, the safest source is a seat from someone you know personally, whose usage history you can actually verify — not a seat from an unknown seller with no history attached.