I don’t know whether it’s the right section to ask but I have a feeling this iTunes code was used up. Even more suspicious is that this “guy seller” may not be legit as he recently only signed up in December 2018 as a seller. I tried using the code he provided but it does not work.
Edit: These are the image evidences.
Final Edit: “guy seller” was genuine enough to refund my whole amount so thank you for him. If the code worked, none of this would happened. Consider this closed/locked/solved.