Is it permissible to add a profit when purchasing electrical appliances for others without informing them?

An agent is not permitted to increase the price at which he purchased an item in order to gain a personal profit, as this constitutes a betrayal of the principal and an action contrary to his interests. Whoever commits such an act must offer sincere repentance and return the money he acquired to its rightful owners, whether with their knowledge or without it if he fears a problem. He is, however, entitled to request compensation for his effort.