So you invest $60-70k upfront and then have a good stream of income coming in for a couple years on each car. Obviously collections would be an issue with some people, however you own the title and can retain a key for the car as part of the contract - so hiring out the work to a repossession company would be fairly straight forward for people who default.
Would be easy to draw in customers via craigslist: '2005 Nissan Pathfinder - We finance EVERYONE!'. So you wouldn't need too many cars on the lot because people coming would know exactly what they want.
I think the legal paperwork will be the hardest to get the business running, however I really like the idea. Has anyone ever tried something like this?