If you're looking to get a bargain from someone on Craigslist, then go in with a smile. It turns out flattery will get you a better bargain when dealing with people selling their own stuff.
Here's the logic -- when people sell their own stuff, they feel a sense of impending loss, so they inflate the value of the product. It's called the "endowment effect," and flattery seems to break down their guard.
Conclusion: If you're interested in buying a second-hand car, it would be a good idea to compliment something about the owner. For example:
"wow, you have a beautiful family"