Sander They bought a game key from a third party. This third party (presumably the shop that they bought it from) then did a chargeback on the original transaction where the game key was acquired, rendering them unable to actually use the key that they paid for.
Dion-Yellow-Shark With an iDEAL transaction, it's just like with a regular bank transfer. There's no way to do a chargeback from your end / from the sender's bank. Maybe bunq support could assist you in contacting the other bank to inform them that their client is a scammer, but chances are that the money is already gone. You could also get the police / a lawyer involved if you deem that appropriate. However, if the identify of the merchant is unknown to you, the chances of success through this route are probably low.
In general, never use iDEAL or bank transfers to pay people that you don't trust a 100%. And definitely not if you're not completely sure of their identity, because that makes it very easy for them to take advantage of you. It's better to use credit cards, PayPal or something like this where you at least have a chance at getting the money back if you can prove that the other side is not holding up their end of the bargain.