Critter Avoid any Verizon or AT&T phones. All phones sold from them have locked bootloaders. Even if it says it's unlocked, that just means it's "carrier unlocked" which means you can use any sim, but the bootloader will still be locked.
I would look for a phone that was originally purchased directly from Google (Never associated with any carrier) and specifically ask if it is bootloader unlocked (not just carrier unlocked.) Keep in mind unlocked devices are more desirable and so they may be less common on the used phone market.