Is it permissible to buy an item from someone at its old price, knowing that its price has increased and they are unaware of that?

The seller must show the change in market price. It is not permissible to buy at the old price without his knowledge, because that is a forbidden deception (ghabn). Deception is forbidden because it is a form of misrepresentation and fraud. This prohibition is not limited to a specific form but includes all forms of fraud and deceit. Deception is of two types: minor and severe. Minor and severe deception are determined by custom and common practice, or, as some say, by one-third of the price. This determination refers back to the local custom and practice. A Muslim must not practice deceit in buying and selling.