How to completely delete data from iPhone 6 before selling?

Try to take back up of your iPhone data's in a computer by connecting it via USB cable for transferring them to another device later.

There are two different methods for it, some may suit one or other to device accordingly. Here you go for it.

The first one is,

Go to main Settings icon
Select the General option
Tap the Reset button at the bottom
Tap the blue 'Erase All Content and Settings' option
Click on 'Erase iPhone' (red color) option in the next small window to confirm it.
Try to check wether the phone was restored.

If it doesn't work well, then try the following URL guideline http://www.iphonefaq.org/archives/971706

If still problem exist then you may raise a question in Apple's iTunes website., directly.

You can either use the data deletion from the iPhone setting itself. And another option is to use the external apps. As you mentioned yourself, the Apple's official deletion maybe easily used to recover the data. In this case it is better to use external apps.

There are some of the apps that allow you to remove the data from the iphone or iOS based devices.

1. iOS Data Eraser : The app can be installed on any iOS device. The app allows you to securely clean any iOS device. You can permanently remove the content inside the iOS device. And the data removed will not be able to recovered from any recovery tool.

2. iShredder : This is another app that makes use of shredding option. So basically your files are overwritten by a large file. And this way the earlier data becomes not readable. This app can be installed on any iOS device including iPhone.

3. Remote Wiper : This app was designed for the users who like to remove the content from their stolen iphone. This app can be used from ipad or any other iphone device. You just have to make use of the wiper app on both the devices. And from this way it can be remotely wiped. The wiper cleans the device once it gets connected with the network.

4. Safewiper Data Wiper : This app is similar to remote wiper. It is designed to clean the device from remote location. You can also clean the device locally. This way you can make use of both remote and in-house disk wiping methods.

There are many other apps in iTunes store that you can find for file shredding on iOS device. Make sure that you take backup of your data before you use any of these apps.

The standard procedure for completely resetting your iPhone would be to follow the below mentioned steps -

1. If you have paired your iPhone with any other Apple device, unpair all of them.

2. Back up your device.

3. Tap on Settings->iCloud. Scroll down and Sign Out. If you have a device with iOS 7 or prior, tap on Delete Account.

4. Yap Sign Out again. Now tap on Delete From My iPhone. Enter your password when prompted.

5. Navigate to Settings. Locate General > Reset > Erase All Content and Settings.

6. If you have enabled Find My Phone feature. you may need to enter your Apple ID.

7. Tap Erase.
That's it! You have successfully all the content from your phone.

Please note that you should nor delete any content from your phone when you are signed in to your Apple account. Doing so can delete some or all content from iCloud as well.