I talked to two separate people today who found cars for sale on Craigslist and got ripped off.
I firmly believe people are essentially good-hearted but here is the most helpful tip with regard to buying things from private parties:
Actually take possession of the item at the time of sale.
No Goodies = No Money.
Incidentally there are other important things about buying cars. For one you will want the title and a bill of sale. It'd also be nice to have every piece of ID you can possibly verify for the selling party in case it all goes bad and you need to track them down later.