What is the ruling on an intermediary (broker) between the seller and the buyer, what is the form of his contract, from whom does he receive his wage, and is he liable if the sold item perishes?

The broker is the intermediary between the seller and the buyer. Their work is Islamically permissible, and they are allowed to receive a fee for it. Brokerage is established by any wording that indicates it, as there is no specific formula for it. So, if a merchant says to a broker, "Sell this, and you will get such-and-such," or a buyer says, "Buy or find for me such-and-such, and you will get such-and-such," or the broker says, "I will sell for you or buy for you such-and-such, and I will get such-and-such," all of these establish the brokerage and entail its consequences.