I personally don't like used laptops since you have no idea what's lurking inside. You can also get a perfectly capable brand new laptop for $500 these days that will last a few years at least. Why save $100 - $200 for years old technology when you can get brand new for not that much more.
A refurb directly from the manufacturer (Lenovo, Dell, Apple, etc.) or a major retailer like TigerDirect, Canada Computers (not sure if they are into refurbs) is better (return policy, someone's actually done a proper install of Windows, etc.) if you must have a used laptop.