It's a totally different business model to this but http://www.webuyanycar.com/ is a pretty big deal here in the UK. You get a valuation, take your car to them, they buy it. To my surprise it worked pretty well too and I got almost as much for my car as I'd have realistically got waiting for months on a classifieds site or whatever. I imagine the way the British used car market works has a big impact on the idea actually working though.