In general I recommend buying new unless one's financial situation prevents that from happening. There are great values out there for Macs that are just beginning to show their age. These older but still quite functional Macs can do most of what an entry-level computer user may need or want to do. Often these older computers are just sitting around unused and can be had for a song or two.
User groups are a good place to begin.
The more local the BBS, the better. For the most part. AOL's online classifieds generally feature the most current pricing on both new and used Mac hardware. People often can find prices lower than even the lowest street prices by shopping around online. New computer introductions can cause used hardware values to drop by hundreds of dollars.