A sugestion about, where I can sell items, like a steam wallet gift card.

Ebay support always takes the buyer's side in a dispute.

You should have a receipt with the activation. Of course the buyer could always state that you probably have used it before and just put another scratch-off panel on it, but otherwise you could have done that for real. However if you don't offer PayPal as payment method, it can't be reverted automatically.
I often buy these at Ebay, since you usually get them with a small discount, but in my role as a buyer I obviously prefer PayPal in case I get scammed by a seller. But I noticed that many only offer bank transfer.

However the possibility to get scammed is always there with activation codes. To avoid that you would have to meet with the buyer. Then you should advertise for it in a classified ad.