Is it permissible for a jewelry merchant to buy gold from his widowed and elderly female neighbors at the same price—lower than the market price—that he buys from other customers, or is it incumbent upon him, as an agent, to seek the highest price for its sale?

The merchant must sell the gold at the highest possible price to his elderly female neighbors because he is an agent and a trustee. The fundamental principle is that an agent works for the benefit of his principal, and it is not permissible for an agent to buy for himself from his principal.