How do I deauthorize a computer I no longer have?
“iTuenes: How do I deauthorize a Computer I no longer have?“
365-Day Waiting Period to “Deauthorize All”
Being this computer is no longer accessible to you at the moment, you cannot deauthorize it directly through iTunes (or the TV app or the Music app) on the computer, right away. You’d have to wait until the 365-day waiting period is over to successfully can deauthorize all devices remotely (meaning with them not in your hands).
Where to Go to Rid of this Device?:
Go here and perform as instructed: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support