I had a similar experience with eBay. The problem is that “unlocked“ typically refers to being able to use it on a different carrier—not related to the bootloader.

So I bought an “unlocked” Pixel that had a locked bootloader. Returned it. Felt bad because the seller correctly classified it.

Ultimately I found an eBay seller (thegizmotrader) that explicitly lists the bootloader as unlockable.

When buying secondhand, I suggest looking for Pixels not associated with any particular carrier (as are sold by Google online store), and especially not Verizon.