Selling your old phone? 8 things to do right now

Backup WhatsApp chats.

Export Contacts. 

Backup photos/videos to Google Photos, iCloud.

Turn off Find My Device in Settings. 

Factory Reset your phone.

Boot it, and this time, do not add your Google account. Simply skip everything.

Now, again Factory Reset the phone. This will ensure double protection as no hacker can retrieve your data now. 

After following these steps, you are good to go. Go ahead and sell your device without any worries.