Is it permissible for me to sell an item—for which its owner appointed me to sell it—at a price higher than the price he set for me, or at a price higher than the price I intend to give the owner of the item, knowing that the increase will be for me? And what should I do if I have done this previously, and do I owe expiation?

If a person commissions you to sell his goods, it is not permissible for you to take the excess for yourself except with his permission, because an agent acts in the interest of his principal, and any increase reverts to the principal. If you do so, you must return the unauthorized increases to their owners or seek their forgiveness. Otherwise, you should give them away in charity on their behalf, and inform them later if you manage to reach them. However, if you are a broker and you agreed with the seller to take whatever exceeds a specified price, or to take a known commission, then the entire price belongs to him, and you get your agreed-upon commission.