I will bet a lot of the listings you are looking at really are OEM unlocked, they are just not clear about it. I bought a refurbished pixel 7 pro on Amazon in 2023 for $234. The listing just says "service provider: unlocked". I installed graphene on it.
That said, I see a subsequent review on this listing commissioning that the received phone was not carrier unlocked, so I guess you have to be prepared for some trial and error and returns.
I would broaden your eBay search to just "unlocked" and then maybe message sellers to verify the kind of unlocked state.
Possibly something here is useful: https://discuss.grapheneos.org/d/13146-any-way-to-tell-if-a-sealed-in-box-pixel-is-oem-unlockable