Depending what you are looking for Craig's List would be a good place for larger machines. Certain woodworking outlets will also take in used equipment and sell for good prices.
As far as smaller tools look online for reconditioned tools. They carry a warranty and have been brought back to factory new specs. Best part is they are cheaper than new. If you pay attention sometimes reconditioned tools will go on sale and you can save some big bucks.
Look at CPO, Big Sky Tool, just to name a few.