How do I prepare my MacBook Air for sale to another?

Click here and follow the instructions. If the computer originally shipped with 10.6.8 or earlier, when you reach step 8, use a Mac OS X install DVD or plug in its original thumbdrive instead of the recovery system.

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This support article describes what to do before selling or giving away your Mac. --> What to do before you sell, give away, or trade in your Mac - Apple Support