Felipe -
Cars used to be sold via classified ads in newspapers, until Craigslist came around. Craigslist removed a huge source of revenue - classified ads - away from newspapers. That, and the fact that nobody receives newspapers anymore.
I would say that any classified car site, even one focused on Utah, has to compete with Craigslist. How to do that? Craigslist has the audience of people looking to buy cars, and a ton of people who want to post cars for sale.
Craigslist, due to it's minimal, almost text-only design, loads really fast. It allows people to upload images, and list a price. It makes it really easy to find cars.
Whatever this site that you're building for a client needs to do everything that Craigslist can do, but more.
One thing I don't love about Craigslist is that you can't search specifically by car model, year, type to hone in on the results.
A better search option would be really helpful.
That said, if you're just starting out, you'll likely face an inventory volume problem - so you don't want to have too many empty categories.
Another thing that doesn't work as well for Craigslist is that the quality of car images tends to not be great. Probably this is my bias, because I'm a professional photographer. But an iPhone shot of a car just doesn't work well to sell a several thousand dollar car.
Perhaps one idea is that to launch the site, anyone who wants to list can drive the car to a location in Salt Lake City, or somewhere else, and have someone on the classified team take professional photos of the car, against a nice backdrop, and also give nice detailed macro close up images of the cars and trucks for sale. Including one of the open engine compartment, the odometer, etc.
One other idea for functionality: allow users who come to the site to get an email alert when a car matching the description of what they're looking for (i.e. I want to buy a Toyota 4Runner w/ a manual transmission).
You'll obviously want to do tie-ins to Facebook, Twitter and Pinterest, to allow people to pin something they want, and share when they list something.
That said, posting on Craigslist is free. And it's touch to compete with free. (Craigslist makes their money for job postings, and everything else is free.)