Is it permissible to buy a commodity and resell it at a higher price without informing the buyer of the original price?

There is no objection to a merchant selling his goods for more than he paid for them, and he is not required to inform the buyer of the original price. However, he must be truthful and avoid lying, cheating, or betraying.

Indeed, a severe warning has been reported for whoever swears falsely to promote his goods, that Allah will neither speak to him nor look at him on the Day of Resurrection.