Wipe iPhone/iPad/iPod touch clean
There are a few steps to do:
Switch off iMessage in Settings > Messages
Switch off FaceTime in Settings > FaceTime (iPod touch 4th Generation or later, iPhone 4 or later and iPad 2 or later)
Delete iCloud in Settings > iCloud > Delete Account
Finally Settings > General > Reset > Erase All Content and Settings.
Un-register your device: https://supportprofile.apple.com
Now you can sell/give.