In the context of an online marketplace "Unlocked" means "Not carrier locked". This has _nothing_ to do with the bootloader.

Well, it has something to do with it. In the context of Pixels, carrier‐unlocked phones always allow the bootloader to be unlocked, Verizon‐locked phones never do (see, e.g., https://discuss.grapheneos.org/d/24254-no-way-to-use-my-veri...), and for other carriers it varies but often requires additional manual fiddling. To quote the GrapheneOS FAQ:

> Devices sold in partnership with specific carriers may be locked by the carrier, which will prevent installing GrapheneOS. This is primarily an issue with US carriers and isn't common elsewhere in the world. To avoid this, either don't buy a carrier device, or make sure it can be unlocked.

Is there a difference there for a Pixel? I thought those bootloaders have always been unlockable (after carrier unlocking, which should be possible after the contract is paid off).

Maybe. But Swappa confirmed it meant bootloader unlock

If it's not a carrier locked device, what would be the reason for it not to be OEM unlockable once connected to the Internet? This is not about the device being logged into an existing user account, that would trigger FRP which is something different.

Someone can be forgiven for thinking that "unlocked" means "not locked" rather than "locked but with only one lock".

I only buy used devices from online marketplaces/vendors with free returns, as it keeps the incentives aligned such that sellers don't want to hide defects (as it just increases their return rate).

100% this.