The proper way to "clean" a computer before selling it is to zero the hard drive. This will make all your data irretrievable.
Steps for zeroing the drive Disk Utility:
1. Insert your Mac OS X CD-ROM disc or Restore DVD disc, then restart the computer while holding the C key until you see the spinning gear.
2. Once started up from CD or DVD, choose Disk Utility from the Installer menu.
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from disc to access Disk Utility.
3. Choose the entire disk (top icon).
4. Choose erase tab.
5. Click on Options
6. Choose "write zeros".
7. Erase
This will take a bit of time because the computer is physically writing zeros to each block.
You should then re-install the original OS from the system disks and deliver the Mac to the new owner with the system disks that shipped with it.
If you want it to boot as new at startup and create an admin account:
Then, launch Terminal and type:
sudo rm /var/db/.AppleSetupDone
Press Return
Then
cd /
Press REturn
Then:
sudo rm -r /Users/yourname
Press return.
Then:
sudo nicl -raw /var/db/netinfo/local.nidb delete /users/yourname/
Press return
Then, shut down the machine immediately.
-mj