Prepare Your Mac for Sale
Boot from the OS X Installer Disc One that came with the computer. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select the startup volume from the left side list then click on the Erase tab. Set the format type to Mac OS Extended (Journaled) then click on the Options button. Select the one pass Zero Data option and click on the OK button. Then click on the Erase button.
Note: You can skip the Zero Data option if you are not concerned about removing sensitive personal data from the hard drive. If you choose to skip this part of the process then it is possible for others to recover data from the hard drive. The Zero Data procedure will prevent others from getting access to your personal information.
This process will take 30 minutes to several hours depending upon the size of the hard drive. After formatting has completed quit DU and return to the installer. Now complete the OS X installation. At the completion of the installation do not restart the computer. Instead just shut it off. The next user will be presented with the Setup Assistant when they turn on the computer just as it would if new out of the box.
If it didn't come with a Snow Leopard install disk, you never actually owned a copy of Snow Leopard, and thus you cannot reinstall Snow Leopard. You'd have to buy a copy for $30, then be sure to include that (as well as the original Leopard disks) with the machine when you sell it. Note that it would technically be illegal to sell it with Snow Leopard on it but not include the disk, so the original seller did the wrong thing... though at least they included the original disks, which many don't bother with.
Hold c and reboot off the 10.5 disk
Second screen in is a Utilities Menu > Disk Utility
Select your boot drive hardware name, click Erase > format HFS+ (default) > Security Option > Zero All data
Wait a hour or two depending upon it's size, it will overwrite the entire drive with 0's and erase all your data completely. (you can do 7x if your paranoid, 35x is just insane and takes days to complete)
When finished quit and install OS X 10.5, shutdown and just leave it for the next person.
Let them buy their own 10.6.