If you plan to give away, sell or trade in your Mac, you should de-authorize your iTunes account on the computer first, as this removes its access to content that you bought from the iTunes Store, iBooks Store, or App Store, including things like music, movies, TV shows, apps, and books.
Apple puts a five-computer limit on an account for accessing iTunes protected content, so it’s worth remembering to deauthorize before you part ways with a computer, but of course that might not always be possible. Say your Mac stops working or gets stolen, for example. What then?
Fortunately, if you no longer have access to the device you want to deauthorize, you can still do so by following the steps below on another computer. The process deauthorizes all computers associated with your account, but also lets you re-authorize the devices you still own.
Read more at MacRumors.com