Is selling merchandise at a price significantly higher than its usual price to a stranger—with their consent—considered unlawful, and what is the Islamic legal guideline for profit?

Selling and other transactions are based on mutual consent. The completion of a sale contract and the fulfillment of its conditions result in the transfer of ownership of the commodity and the price. There is no specific limit for the profit of the seller or the buyer as long as the sale is permissible and free from deception. Therefore, it is permissible to sell an item for many times its value or to buy it for less. However, it is preferable to avoid egregious exploitation, due to the saying of the Messenger of Allah, peace and blessings be upon him: "May Allah have mercy upon a man who is lenient when he sells, when he buys, and when he demands his rights"; and to avoid disputes regarding the binding nature of the sale if it involves unusual exploitation.