Before you sell make sure you do a full factory reset. Info below, copy and pasted from Apple
If you still have your iOS device
Before you sell or give away your iOS device, make sure that you've removed all of your personal information. Follow these steps to protect your data and get your device to its factory default state for the new owner:
Back up your device.
Go to Settings > General > Reset, then tap Erase All Content and Settings.
This will completely erase your device and turn off iCloud, iMessage, FaceTime, Game Center, and other services.
If you're using iOS 7 and have Find My iPhone turned on, your Apple ID and password will be required. After you provide your password, the device will be erased and removed from your account so that the next owner can activate it.
Contact your carrier for guidance on transferring service to the new owner.
When the device is turned on for the first time by the new owner, Setup Assistant will guide them through the setup process.
Important: Do not manually delete contacts, calendars, reminders, documents, photo streams, or any other iCloud data while signed in to your iCloud account, or the content will also be deleted from the iCloud servers and all of your iCloud devices.