Since I have just been through this, I'll answer. Anyone correct me if I'm wrong
Your best bet would be to buy directly from Google store as it will be bootloader and carrier unlocked
Carrier unlocked is not the same as bootloader unlocked so you could buy a carrier unlocked pixel but it's not able to unlock the bootloader
I would only buy second hand if I could see the device in my hands to confirm you can toggle the unlock and that it's not greyed out before you buy it, otherwise I'd buy from Google store to be sure
P.s. I bought direct from Google to be sure