With your phone backed up and all of your accounts removed, factory reset it by following these steps:

1. Open the Settings app. 

2. Select General. 

3. Scroll to the bottom and select Reset. 

4. Select Erase All Content and Settings. 

You'll need to enter your phone's PIN code along with your Apple ID password to remove the phone from your account. In the past, you had to disable Find My iPhone (now called Find My) before you could reset your phone. Now, however, Apple prompts you for your Apple ID password during the reset process so it can turn Find My off for you. 

After entering your password, the screen will go dark and the Apple logo along with a progress bar will show up. A few minutes later, you'll be greeted with a screen with "Hello" in different languages flashing on it, indicating the reset was a success.