Here's another benefit - if the seller allows it, you may get a machine fully loaded with software. I bought a used 2-year-old Mac that was loaded with over $1800 of extra software (like the latest Adobe Photoshop suite, Windows 7 and more). And it cost me less than half of a new machine, but fully loaded with max RAM and features. Nothing wrong with not being cutting edge, let someone else pay for getting the latest and greatest.
Take a look at the above post. . .