Whether you sell your iPhone by yourself or sell it into a third-party company, keep in mind a few things to make sure your crucial data is safe:
Backup your crucial data: Make sure to backup your iPhone including contacts, photos, videos, and apps using Apple’s iCloud service or a third-party cloud storage service. Just, connect the device to your computer and open Image Capture or iPhoto. Select all pictures and then click Import All.
Turn FaceTime off: The next thing you should do is to turn off FaceTime. You can do it by Open the Settings app > FaceTime > switch to off.
Turn iMessage Off: The next thing you should keep in mind is to turn off iMessage. You can do it by Open the Settings app > Messages >iMessage> switch to off.
Log out of Apple ID: Another crucial thing is to log out of your Apple ID. You can do this by following these instructions: Open the Settings app >App and iTunes Stores > tap Apple ID and sign out.
Erase your data: Once your data is safely backed up, it’s vital to erase your data including photographs, files, messages, etc. Don’t skip to sign out of all apps, services and connected accounts also.
If you have no clue where to sell your iPhone, then I have some wonderful suggestions for you:
First, remove your all kind of webmail from your phone. Remove iCloud completely. Do factory reset your phone 3-4 times so that everything got erased. Check completely every setting & folders like gallery, calenders, contacts etc. dob’t leave any kind of your personal data.
Before doing these all things make sure that you have completely backuped your all personal data.