Boot from the OSX DVD
In the Utility menu go into Disk Utility
Choose Erase (optionally, choose Security Options and choose Zero Out Data which will overwrite your data with zeros but takes much longer)
Do an OSX install from the DVD
You probably already know this, but If you plan to sell the software, you must include the CDs/DVDs and the license with it. Otherwise, selling software 'preinstalled' on a drive without CDs and transfer of license is not legal.
The fast and non-reliable totally insecure way is to create a new User with admin privileges, and then delete you own User folder.
Arrr, just t' amplify what our good matey Canadaram said, that "Zero Out Data" option will take a Long, Long Time. Like a mightily long time. Hours and hours.
Our store used t' follow his recommendation, but at the end o' the Os X install, we'd let the wench reboot and boot up t' the registration screen, wait for all disk acti'ity t' stop, then power off (hold ye olde power button for 4 seconds). This left the wench in an "as-new" state (will restart the "hello/registration" wizard on boot). Aye.
Easy quick and reasonably secure. Create new admin account. Trash the old account /Users/"OldAccountName(s)" and then do a "secure empty trash".
Next do a format from the Install Disk, Disk utility. A single partition is fine. Reinstall just the basic OS and as mentioned previously include the Install disk with the sale.
For the really, really, really paranoid. After the secure empty trash step. Boot the old computer into target mode, connect to a newer Mac via fire wire. Do a quick erase from disk utility. Overwrite the entire HD with copies of very large jpg files, unmount the target drive then disconnect the firewire cable and reboot the target computer from the install disk. Finish this up as before with the format and install. Any one trying to scavenge data will see only those jpg files.
This last bit is a lot faster and provides greater security than just zeroing the HD.
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