you simply have to trust that the seller from whom you are buying actually gives you a real key. Even when you get a real key, there is no way of verifying how the seller acquired those keys.
Keys not working I have bought several games from Kinguin without any problems. But this time I have bought Zoo Tycoon Ultimate with several Keys inside (total €60.79). Only the game key is working but the keys of the DLCs are not working. I have made a ticket, sent messages to the ...