When selling a personal computer, it is advisable to erase as much information as possible to protect your privacy and make the computer more appealing for the buyer. The best way to do this is to reset the computer to its factory settings. With an Apple iMac, you can do this using the system disks that came with the computer.
Insert the System Install or Restore DVD that came with your computer into the optical drive.
Restart the computer while holding down the 'C' key. This will reboot your computer using the disk you have inserted.
Select the language in the screen that appears (for example, "Use English for the main language") and click the arrow to continue.
Choose "Disk Utility" under the Utilities Menu on the screen that follows.
Select the icon representing your hard drive in the left panel of the screen. This is usually called "Macintosh HD."
Click the "Erase" tab in the right panel of the screen.
Choose "Security Options" on the screen that loads.
Select the box that says "Zero Out Data" and click "OK."
Click "Erase." When prompted to confirm this action, click "Erase" again.
Select "Quit Disk Utility" from the drop-down menu that appears when you select "Disk Utility" under the "Utilities" menu.
Click "Continue" on the Mac OS X installer screen.
Select the hard drive on which you wish to install the operating system.
Select "Continue" once the installation is complete.
Restart your computer.
Mac OS X 10.4 or above is needed to use this method.
Once you erase your hard drive, your files can no longer be accessed. Remember to back up all important files and information before proceeding.