The only thing you need to do before selling an iPhone (or iPad) is go into Settings->General->Reset->Erase All Contents and Settings. This will prompt you for

The only thing you need to do before selling an iPhone (or iPad) is go into Settings->General->Reset->Erase All Contents and Settings. This will prompt you for your iCloud password and remove the iCloud lock and wipe the device. Source: sold at least a dozen iPhones and iPads since iCloud lock has been a thing.