Power Max - These folks sell both new and used Mac stuff. They sell a lot more than computers, but their selection for Mac stuff isn't bad. MacConnection MacMall MacZone Sun Remarketing - These guys specialize in older Macintosh stuff. They have a large variety of merchandise, but they can be kind of pricey. Another company that specializes in older Mac stuff.
Same comments apply.
Preowned Electronics - Another place that specializes in older Mac stuff. Their prices aren't as bad as MacResQ or Sun Remarketing, but they're still not great.