OK, by this point, you've taken care to ensure the security of your data. But while it might be clean on the inside, the buyer, assuming you have one, isn't going to like getting a machine that's covered with fingerprints and encrusted with pizza remnants.
So be considerate and give your old machine a good external cleaning, using a soft damp cloth and compressed air to get particles out from the crevices in your keyboard. If it's a desktop, you can also unscrew the door on the case and use the compressed air to clean out dust bunnies on the inside. For a laptop, carefully turn the machine upside down and gently shake out the crumbs and dust in the keyboard.
Finally, use a soft glasses cloth to wipe away smears and fingerprints from the screen. You'll feel better that you did [source: Asch].