If you want to wipe out the contents completely, a clean install won't do it. You would need to wipe the drive using the included Drive Setup in the Utilities folder in the OS 9.1 CD. Then just perform the install as normal once the drive is formatted.
A clean install will only rename the old System Folder and create a new one with only the Apple-supplied Extensions and Control Panels. All the other files will still remain and the old System Folder will be renamed to "Previous System Folder." I don't think you want this available to the buyers of your computer.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1