This is a great post, but it's a bit out-dated since Leopard (Mac OS 10.5) came out and did away with NetInfo Manager. The procedure for making a Mac OS X system look like new for Leopard and Snow Leopard is as follows:

1. Boot into single user mode (hold command-S during startup)
2. mount -uw / (this is to mount the filesystem as read/write. It is read-only by default)
3. cd /var/db/dslocal/nodes/Default
4. there will be many files in this directory, which you can see by typing "ls" or "ls -lh". Locate the files that represent YOUR user accounts and end in .plist and delete them by using the "rm" command. For instance, if my log-in name was "michael", I would type "rm michael.plist"
5. rm -r /Users/<your user directory>
6. rm /private/var/db/.AppleSetupDone (this makes the initial setup routine run again)
7. exit

At this point, your machine will exit out of single user mode and continue the boot process and should bring you to the initial setup utility. At this point, it is safe to force-powerdown your computer.

Note... the unix "rm" command permanently "removes" any file directory that you tell it to without asking for confirmation, so double-check your typing before pressing <enter>. However, if you are selling your computer, you probably want to use Disk Utility to erase and zero out your disk before you re-install Mac OS X and install the updates to make sure your data isn't easily recoverable.