Is money earned from selling goods at a higher price to those who do not possess credit cards, by using an Islamic credit card, lawful?

It is permissible to use a credit card for online purchases if it is free from Shariah prohibitions. As for providing goods to those who do not have a credit card, there are two scenarios:

First: That you purchase the commodity first, then sell it for a profit. This is permissible. However, it is not permissible to sell a commodity before possessing and acquiring it, due to the Prophet’s (peace be upon him) saying: "Do not sell what you do not possess" and "If you buy a sale, do not sell it until you take possession of it."

Second: That your role is merely to finance the purchase, by paying an amount to the buyer and then reclaiming more than that, without purchasing the commodity for yourself. This is forbidden because it is an interest-based loan (riba), and riba is among the major sins.