It is permissible to sell iTunes cards for the same amount as is written on the card or for more or for less. It is also permissible to sell them for instalments to be paid at a later date, because they are not the same as cash in which it is stipulated that they should be of the same value and the exchange must be completed on the spot. Rather it comes under the heading of selling the benefit. There is no difference between them and phone cards or cards for internet access and the like in which there are some costs involved in producing the card that the purchaser buys, so these cards are for some benefits and there is some work and effort involved in producing them, so there is nothing wrong with selling them for more or less than the amount written on the card.
And Allah knows best.