You can actually just shut down your system safely when you get to that point. Hold down the power button for 3-5 seconds and the system will power down/shut off.
When the new owner boots the machine it'll proceed directly to the system setup as if the machine was turned on for the first time. Presuming you performed an erase and install.
We all at some point upgrade from our present Macs to newer state of the art Macs. For example upgrading from a 17" PowerBook G4 to a 17" MacBook Pro.
Macs have a high resale value and people are always looking for a second system, a system for their children, or looking for a bargain. Remember to include the restore disks, user manuals, accessories, etc., that came with your system when you first bought it.
One of the things that make a Mac special is the high resale value. It is also how many new users, "switchers" experience their first Mac. 9 out of 10 those buyers will purchase a new Mac down the road obviously because of the Mac itself but also because of their buying experience.
They always remember their first Mac and their actual buying experience, so make it as if you were buying the system. What would you consider when buying from a complete stranger and what would you want and how would you want to be treated?
Good Luck!