Thanks for all the detailed responses. This is great help. A bit more info for those who asked: The commute is roughly 1.5-2 miles, all city roads, and only one hill in the way. My roommate says his bike is a 53cm and it fits great. I think I'll look for something in that range.

If you wouldn't mind, a few more craigslist examples would help a bunch. I've started to look twice a day for about a week now and no luck yet. I'll be sure to post once I find something.