I've bought used Pixels on Swappa. You'll want to make sure you get one that has an unlockable bootloader. Carrier unlocked will be your safest bet, absolutely avoid anything Verizon branded. On Swappa, you can message the seller (publicly) and ask them to confirm the bootloader is unlockable.
Enable the developer options menu by going to Settings > About phone and repeatedly pressing the build number menu entry until developer mode is enabled.
Next, go to Settings > System > Developer options and the ‘OEM unlocking’ setting will hopefully not be greyed out.