First things first, if you are selling iTunes cards, you are the seller and the person buying from you is the buyer.
In the case you deliver a valid iTunes gift to a buyer and they claim it “didn’t work”, you can get in touch with Apple through their live chat online to have them look into where and when the card was used. Screenshot the conversation where they confirm it and submit it to Gameflip for review. They will match up the information and if it matches, the buyer will more than likely be suspended permanently for fraudulent behavior and you will receive your money.
Similarly, if a seller sends a fake code, the buyer can find out when the code was used or if it was ever valid. If it was used somewhere else or never valid to begin with, the seller will likely be permanently suspended for fraudulent activity.