Whether you sell your iPhone by yourself or sell it into a third-party company, consider in mind a few things to make sure your data is safe:
1. Backup: Before you do anything to sell your phone, make sure you have an up-to-date backup of your iPhone including contacts, photos, videos, and apps using Apple's iCloud service or a third-party cloud storage service.
2. Erase your personal data: Once your data is safely backed up, it’s vital to erase your personal data including pictures, files, messages, etc. Don’t forget to sign out of all apps, services and connected accounts also.
3. Turn off Find My Option: Apple's Find My app is a security feature that must be turned off before you sell your phone online.
4. Clean up from outside: Your iPhone must be clean not only internally but externally as well. Take a few minutes to wipe it down with a cloth and make sure all dirt, stains and even fingerprints are gone properly.
5. Remove the SIM card: The last thing is to remove your SIM card carefully, which you'll likely need for the new phone to keep your existing number.
By following these important things, you’ll surely sell used cell phones online without any worry.