Hi,
I buy a lot of bikes and bike parts from both craigslist and ebay. Never had a bad experience, but a few I have had to take apart and overhaul and fix. Since I do my own builds it's no big deal - and the fact that they needed some work made the parts a good deal.
I guess you need to know what you are buying - well enough to determine the conditions and the work needed if any. Then you can bargain and make good deals.
Good luck