I've used eBay to buy several used Pixels for GrapheneOS, without problem.

The easy way is to search for ones for which the seller explicitly says "OEM" or "bootloader" unlockable/unlocked (or seller says it already has "grapheneos", "graphene", "calyxos", etc., installed).

For awhile, I came up with some tricks to try to get a better price by identifying ones that were bought directly from Google (rather than through a carrier, which are who has been disabling bootloader unlocking thus far), but decided it wasn't worth the effort.

Of course, you can also just buy a new one from Google.