1) Your's is a listing site you can start with premium listing features giving more visibility to the paid post.

2) Could charge a small fee to the user to get their listing verified from your side. Namely the car condition and other details that they mention on the site.

You could also sell leads to suppliers or you can sell the parts themselves.

The key is to understand what your users want and what other parties who are willing to shell out money want and strike a balance.

start by adding option to "feature" the car in the sidebar or somewhere. just make small changes that add value ( speed up the buying-selling process )