I think machine's advice of starting something like this in a niche would make the most sense. To compete with cars.com or vehix.com is going to take a lot of money. First to buld the site itself, next to maintain the site, and probably mostly for the marketing and advertising required.

A site of that scale is generally not something you would tackle on your own, but would most likely require some heavy backing. That is assuming you'd want to launch on the scale of those other sites.

It would be much easier to launch in a niche, maybe a specific type of vehicle. I like the idea of parts since I think most people going to these existing sites are looking for the entire car.

Parts particularly for older cars can be hard to find and a site where people could trade and sell dfferent parts would probably stand a better chance of surviving and would possibly be easier to launch.

It's still going to involve a lot of back end programming and security and is going to cost a good amount to set up. I think you could get it done under $100k, but we're certainly talking 5 figures at a minimum.