It sounds like you bought a used smartphone that has a screen lock on it. Are you sure that it wasn't stolen? The owner should be able to easily unlock it for you, just give the person a call and ask for the PIN or lock pattern. If they can't provide it, there is a chance that it was stolen and you are in possession of stolen goods (which in itself can be a crime). If it was reported stolen, it should also be blacklisted by the carrier to prevent activation.