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 ...

If you attempt to play or sync content that requires authorisation but haven’t yet authorised the computer you’re using, you’ll get an alert dialog and the means to sign into an iTunes account.

You have an ancient Mac that's authorized with your Apple ID. Said Mac won't connect to iTunes for the purposes of deauthorization. What to do? Throw the game board up in the air, says Chris Breen.