iPhone 6 and 6 plus will re released Friday (like you don’t already know that). Before you sell your current phone there is one important thing you must do in addition to removing all your personal data. You have to turn off “Find My Phone”.
The purchasers can’t reprogram the old phone if Find My Phone is still turned on. Say you sell your iphone to Gazelle. If you don’t turn off Find My Phone and they can’t use the phone, you don’t get the money and you don’t get your phone back.
To turn it off, go to Settings > iCloud, then tap to turn off Find My [device].
Easy but important.