C'est une voiture d'occasion?- Non, elle ou c'est...

It is "la voiture", therefore "elle" and "neuve" both are in the feminine form since they refer to it.
You don't say "c'est.." because you refer to a specific vehicle, in which case you use "elle" instead of the impersonal "ce".

Here is the lesson on this: https://french.kwiziq.com/revision/grammar/when-to-use-cest-or-il-est-elle-est-to-say-it-is

-- Chris (not a native speaker).