I sell second hand car seats all the time. The things to look for are:

* Check the harness straps and tether straps for fraying (don't buy if there is any).

* Always ask the history of the car seat (how old is it? has it been in any accidents?)

* Check the plastic parts for signs of stress. The plastic is designed to show stress (i.e from an accident), it will have faded looking lines/marks if it has had stress. You can see this by turning the car seat around and looking at the back.

* Don't buy anything older than 10 years old (from the manufacturers date, not from when the original owner bought it) or it no longer meets the Australian standards. All Safe N Sound car seats have a date sticker on the left-hand side (your left!), if it has been removed, look for a date stamp imprinted on the plastic at the back of the car seat. The year (i.e 04, 05, etc.) will be printed inside a circle that will have a tiny arrow pointing to the month it was manufactured.

* Look closely and make sure it feels structurely sound, it should feel nice and firm and not wobble excessively, etc.

* If you intend to use it for a newborn, make sure it still comes with the padded newborn insert, otherwise it is not safe to use for your newborn. You can't use alternative ways to snug them in (like a blanket or towel) as it is not deemed as safe. The inserts can be difficult/expensive to source.

HTH