I have heard the same thing that get a car seat new or one you know the history about. They also have a sticker on them that tells you when they are expired. Then I took it to the county because they offer a service if showing me how to install the seat and talked about all the safety to go with it.
Used car seats are never a good idea. Thats 1 thing you should always buy new. It could've been in an accident, which could make it not properly do its job for safety, may be expired(it should say on the seat somewhere) or recalled. Even minor accidents where u would never know the difference as a used buyer are not good for car seats.