Q: I am planning to sell my AT&T approved unlocked iPhone 4. What reset command should I use to prepare it for sale to a stranger?
A: The process for doing this is actually quite straightforward for any iPhone or other iOS device, regardless of whether it’s unlocked or not.
Simply go into your Settings app and choose General, Reset and select the option to Erase All Content and Settings.
If you have a passcode set on your device, you will need to enter it before proceeding, after which you will be asked to confirm—twice—that you actually want to continue and erase everything on your device.
This process should actually only take a few seconds to complete, after which the iPhone will restart and display the standard iPhone setup screen. At this point you can simply turn it off and pack it up for resale.