Heh. My company provides cars.com with most of its data, so I have direct access to their db. ;) However, because our business model is predicated on aggregating and then reselling automotive data, it's unlikely you could get access for free. Still, if you think you'll ever want to go commercial, you might contact us: (DMI) Digital Motorworks Inc.
Craigslist has a ton of car postings, but it's not structured in any way, will be very hard to parse.