Pictures and information/links. Please know what your selling. And be prompt in your responses to buyers.
In my craigslist experiences I have met some real shady strange people, and I have met some down to earth real people. My most recent purchase was a 115 I had to go to some guys house to get. I don't like going to peoples houses or them coming to mine, but it happens. I ended up spending 2 hours playing bass with this guy and having a great time jamming.
But for the most part, I put caution before everything, meet at a public place, and bring a friend. Just in case.
Oh, and if you're listing, be reasonable in your price. Its craigslist, people aren't going to pay new product pricing for your used item.