If you want to be completely sure that all traces of your data is removed, both of the "prep for sale" guides (here and here) that I came across recommended wiping your machine's drive and reinstalling OS X. The good news is that you can accomplish this without purchasing or installing any additional software - just back up your data and use Disk Utility to wipe the drive.
Apple Toolbox's guide recommmends:
- Back up your data. Using Time Machine, or another backup option, make sure your backup is current. Once you complete these steps, there
will be no way of retrieving anything.
- Boot off of your OS X disk. To do this, insert the OS X disk that came with your machine, reboot your computer, and hold down the C
key while the machine is booting up. Open Disk Utility. Once your
machine has booted off of the disk, and you have selected your
preferred language, go to the Utilities menu and select Disk Utility.
Once Disk Utility is open, proceed to the Erase tab.
- Enter a name for the formatted drive. On the Erase tab, enter a name that will be used once the drive is erased. By default, a Macʼs
hard drive is named “Macintosh HD.” Since you are preparing to sell
the machine, this is a good choice.
- Select the security level. Below the name field, you will see a button that says: “Security Options.” Once selected, you will be
presented with a number of different options. These include: “Do Not
Erase Data,” “Zero Out Data,” “7-Pass Erase,” and “35-Pass Erase.” The
first option is essentially the same as dragging a file to (and then
emptying) the Trash…except that it does it for every single file on
the hard drive. Ultimately, however, the data still remains. The other
three options erase the data and write zeros over the entire hard
drive once, 7 times, or 35 times depending on the option you choose.
For most people, the single “Zero Out Data” will be more than sufficient. If your machine has particularly sensitive information
on it, such as company financials, trade secrets, or the next great
novel, you may want to go with the “7-Pass Erase.” This option is
considered secure enough to be the standard procedure for erasing US
Department of Defense computers. You can rest easy using this option,
while the “35-Pass Erase” is essentially overkill for all but the most
extreme cases. Once you have selected the appropriate option, click
the Erase button.
- Install OS X and reboot.
However, the steps here will not leave your existing software in place.
If you're interested in leaving the software and OS X in place, deleting all of your data files, and use Disk Utility to wipe the free disk space. Steps from Apple Support:
Erasing free disk space does not erase the other files on your disk.
- In Disk Utility, select the disk or volume in the list with the free space you want to erase.
- Click Erase, then click the Erase Free Space button.
- Select an option, then click Erase.
You may also want to create a new admin user, delete your old account(s), and run the Disk Utility routine to erase free space used by your account(s).