Follow these steps before you sell your iPhone
Image Source : FileFirst transfer information to new device then remove information from old device
Image Source : FileDon't manually delete any iCloud information while signed in. This would delete content from the iCloud servers
Image Source : FileIf changing phone number with new device, add additional trusted phone numbers to account so you don’t lose access
Image Source : FileIf you paired an Apple Watch with iPhone, unpair your Apple Watch
Image Source : FileIf you’ve purchased an AppleCare plan for your device, cancel your AppleCare plan
Image Source : FileIf you're switching to a non-Apple phone, deregister iMessage
Image Source : FileRemove your old device from your list of trusted devices
Image Source : FileNext : Top smartphones under Rs 30,000 with 6000mAh battery