They make the bulf of their money from car dealerships. On aveerage dealers spend $500-$800 a month to list all of their inventory, they also make money from private seller listings. It would cost around $100,000 to build the site and at least another $100,000 to market it and even then you will have a tough time getting business. autotrader.com, cars.com, carsoup.com and a few others are who you will be competing against.