Bought Apple Watch.

Unfortunately, there is nothing else that can be done. Check with Facebook to see if they have any alternative for you to file a complaint for a refund, however short of that, there is no way to bypass Activation Lock. No one here, nor Apple can assist you with that, only the original owner.

There is NO other way to unlock watch without credentials of existing, you must contact seller who sold watch to you. Apple will not be able to support you in this regard.