Unless it's stated as unlocked, then more than likely it would be locked to Cingular. It looks like the average price of it would go for about $200. However, I've seen some on Craigslist that go for $150. I don't think you would want to spend more than that.
If you're going to deal with a transaction through Craigslist, test out the phone thoroughly. If you feel there might be something wrong with the phone while testing, you can always back out.