Can you use Apple gift cards to buy a phone from online?
If I buy an apple gift card and send it to someone who says their paying for the phone that they bought, does that mean I’m being scammed?
This article explains the different kinds of gift cards Apple uses and has links on how to redeem them and what can be purchased with each kind. Click on the link --> What type of gift card do I have? - What type of gift card do I have? - Apple Support
If they have already purchased the iPhone then it is too late to change how they paid for it unless they return it and buy it again.