Selling your old iPhone or iPad?
The good news is that you don’t need to worry about de-authorizing it. You can run iTunes on only five authorized computers, but your iPhone, iPad, or iPod is not considered a computer (it’s merely a “device”).
What you do need to do before you sell or donate your old device is erase your apps and data and restore the device to factory settings. You can do that quickly and easily following one of the two methods outlined in this CNET.TV video, “How to erase and restore an iPad.”