1. That's an image, which could have been edited.
2. Even if it wasn't, what does you receiving money prove other than the fact that someone most likely got scammed trying to buy one of those so called gift cards.

There are plenty of ways you are able to prove you have the products you claim you have.
If they're physical, you can take a picture of yourself swimming in thousands of your gift cards.

More than likely, you'll claim they are digital codes. Which in this case, with how much stock you claim you have, you should be able to provide at least one person here with a vouch of your product. I mean, whats one vouch if it means potential increase in customers.

Also, since you were accused of reselling the same gift cards to multiple people on your feedback, what I suggested earlier was a way to ensure you still aren't doing just that. How anyone make sure you aren't going to sell the same gift card to the next person?