I think online car shopping has a huge potential to grow, especially considering most consumers dislike negotiating at dealerships. That's why Edmunds.com launched the "Price Promise" (http://www.edmunds.com/price-promise.html) that locks you into a price before you ever have to speak to a salesperson.

While we're on the topics of online car shopping, I work for Edmunds.com on the API team (docs here: http://developer.edmunds.com) and we provide all kinds of car data for use in your application (and it's completely free as compared to all of our competitors). Feel free to email me (mbock at edmunds dot com) or comment below if you have any questions.