Irony aside, your best and safest bet is to purchase the device directly from Google. You'll have the comfort and reassurance of knowing that nothing's wrong with the device and that you'll be able to use GrapheneOS on it before making your purchase. You'll also avoid any stress and headache with having to return or not being able to return the device. More often than not, when a listing states that a device is "unlocked", it's referring to Carrier- and not OEM/bootloader-unlocked.