I would raise a dispute with eBay, not certain but would expect if you have been sold something you can’t use you should get your money refunded. You will need to prove you were not told of the “lock” until after you made the transaction though, but if something is not working it’s usually described “for spares”, so if that wasn’t in the description it will also support your case.
One last thought, if the seller was the original owner he should be able to help you unlock the device, if he can’t it might mean he’s not and therefore the device could be stollen, another no no I think eBay would not be happy about.