I recently bought a used Nexus 6. It was factory reset by the previous owner which apparently in recent versions of Android means that Factory Reset Protection may cause the new owner to be unable to initialise the phone because a previously signed in account is required.
However, the protection is not very well designed as the voice recognition functionality Google provides in their stock ROM eventually lets you access system configuration to add a new account via a cloud printing option.
In the end I was able to add a non-previously logged in account, fooling the Factory Reset Protection to believe I was a legitimate owner of the phone. The process was simple but I did not document it step by step. This is from my recollection:
Factory Reset Protection is enabled. When booting and trying to setup the phone, you are greeted with the following text:
"This device was reset. To continue, sign in with a Google Account that was previously synced on this device."
Instead of connecting to an identified wifi, choose the "Add network" option in the bottom of the wifi selection dialogue. This lets you add arbitrary text.
- (note that you'll probably need internet access later, so maybe you should just connect somewhere first and then go back to that menu to add a network directly afterwards)
- It appears that the "share" feature of selected text is disabled by Google, which is a method of getting to the system settings in older versions. However, if you instead the little "microphone symbol" to add text via voice, the keyboard changes.
- When the voice recognition is activated, there's a menu (I don't remember off the top of my head what this looked like or what the text was). Anyway, try to click yourself to some help menu.
- Try to print the help. This will notice that you are missing an account for cloud printing et al. Happily press "Add account" or whatever the option is, signing in to some random new Google account.
- Sign in, enable OEM unlocking. Get back into the bootloader, do 'fastboot oem unlock' on your computer and voilà, you're ready to install Lineage OS or whatever less evil than Google's spy crap.
Other guides tell you to download random software and allow unknown sources. That's obviously not necessary. This works for whatever version of Android is on Nexus 6 by default.
h4ck t3h pl4n3t