If you installed Lion simply erase the disk and reinstall Lion. <
^^^ Scratch that, if you installed Lion you own it and you are supposed to erase it before selling the computer.
If you are running any other OS, it is not possible to restore it to factory settings since you would need its original system installation DVDs. As an alternative, you could buy a Snow Leopard install DVD from Apple and use it. However, it will not include any bundled apps that came with the computer (iLife for instance).
Message was edited by: John Galt