How can I erase my hard drive?

To prepare it for sale you should follow these instructions:

What to do before selling or giving away your Mac

In addition to the above:

• If you enabled FileVault, disable it in System Preferences > Security & Privacy.
• "Deauthorize" your iTunes account. Same for Audible if you have one.
• System Preferences > iCloud > de-select "Back to My Mac" and "Find my Mac".
• Sign out of iCloud. Select "Delete from Mac" when it appears.

Then:

• Start your Mac with its original System Installation DVD (if your Mac shipped with one) or OS X Recovery (if it didn’t).
• Remove any Open Firmware password if you created one: select Firmware Password Utility from the Utilities menu and remove it.
• Select Disk Utility from the Utilities menu.
• Remove any partitions you may have created.
• Select the Mac's hard disk icon, then select the "Erase" tab.
• Select the "Security Options" button and erase the disk.

The more "securely" you erase the disk, the longer it will take. The fastest method is sufficient since all but the most expensive techniques and equipment will be able to recover securely erased data.

• When it finishes, quit Disk Utility.
• Select Install Mac OS X from the Utilities menu. Do not create any user accounts.
• When it finishes, shut down the computer.
• If you want to install the bundled apps that were included with your Mac, restart by using your Applications DVD and install the bundled apps.
• Don't forget to include any DVDs that came with your iMac, your AppleCare certificate if you bought it, printed documentation, even the box if you still have it.
• Execute a bill of sale showing the Mac's serial number.
• Once no longer in your possesion, remove the Mac from your devices in My Support Profile.

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erikrhulen wrote:

Thanks. Can I erase the hard drive if I don't have its original System Installation DVD (if your Mac shipped with one) or OS X Recovery?

The quick answer is yes, but your buyer should be made aware of the fact it will not contain an operating system.

Since your Mac is already running OS X 10.8.5, OS X Recovery will work to erase the disk, but a buyer will not be able to reinstall that operating system without the Apple ID used to purchase it (presumably, yours). In other words you cannot sell an operating system purchased from the App Store. That is no longer simply a legal requirement, it's a practical one. A new owner will be faced with continual problems until the Mac is restored to its original state according to the instructions I linked.

According to Apple's terms, you can only sell a Mac with its original OS reinstalled, or none at all. If you were to erase its disk completely that reduces the selling price significantly, even though a replacement OS X disc can be obtained from Apple for a nominal fee.

Buying used Macs is for very experienced Mac users, and since your potential market is comprised of many unsophisticated buyers, you have to make it as boneheadedly simple as possible for them. If you prepare your Mac for sale according to my instructions you will have accomplished that, and the buyer will have every reason to be pleased with the transaction.