Where do you find used equipment?

A call recently came into Concrete Contractor from a reader looking for suggestions on places to find used equipment, specifically walk-behind concrete saws. I offered a few websites and other resources that might be of help, but don't have any practical experience in used construction equipment buying. So if you've had success in buying used equipment online, through a local auction, or by other means, please comment to this post and share your experiences or suggestions.