I have read the FAQ on the GrapheneOS website and I know which phones are supported, but I have also heard that not all of the Pixel phones will allow you to unlock the bootloader or something. And that you need to specifically find an OEM unlocked one.
How exactly can I know that a phone is OEM unlocked before I purchase it? I have found some on ebay such as this one:
https://www.ebay.com/itm/316486347847?_skw=google+pixel+8a
which are "Network unlocked" but I'm not sure if they are also OEM unlocked.