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What to do before selling or giving away your iPhone, iPad, or iPod touch
http://support.apple.com/kb/HT5661
If the iPad was running iOS 7, iCloud: Find My iPhone Activation Lock in iOS 7
http://support.apple.com/kb/HT5818
iCloud: Activation Lock
http://support.apple.com/kb/PH13695
Find My iPhone Activation Lock: Removing a device from a previous owner’s account
http://support.apple.com/kb/ts4515
Buying or Selling a Used iPhone or iPad Running iOS 7? Read This First!
http://www.makeuseof.com/tag/buying-or-selling-a-used-iphone-running-ios-7-read- this-first/