For people new to commuting I do not recommend using craigslist if you can help it. However if you do shop used see if you can find someone fairly knowledgeable to help you out. You might wan to try LBS that sell used bikes or just ask questions on what they have available.

My suggestion would be to find something cheap that fits. Once you get time in the saddle you will have a better idea of what type of bike you want. Also, you never know if you will keep up with it. And regardless of how safe your campus seems, you should always lock up your bike properly.