The rule with c'est vs il/elle is that you are speaking of a specific item. In this lesson one of the examples doesn't appear to follow that rule:
The lesson translates "Is it a second-hand car?- No, it's new." to "C'est une voiture d'occasion? - Non, elle est neuve."
Why doesn't the question use Elle instead of C'est? They are talking about a specific car--i.e. the one purchased by the speaker.