The discount rate for the retailers may differ depending upon the number of people willing to buy such cards. But in the case of cash, it can never be sold at a discount. The reason is that cash is the universal medium of exchange and is legal tender, making it legally recognized and accepted by all people of the country and even outside the country. This universal acceptability means that cash is not exclusive to goods of a single store only.
The versatility of cash enables a person to use it anywhere and everywhere to buy anything, and everything makes the cash never be sold at a discount.