In an intermediary transaction, what must occur for a broker to receive a commission from both parties?

In an intermediary transaction, a broker can receive a commission from both parties only when both parties provide written informed consent. This requirement is in place to ensure that all parties involved fully understand the nature of the intermediary relationship, including the broker's role and any potential conflicts of interest. Written consent protects the interests of both buyers and sellers by ensuring that they are aware of and agree to the broker's dual representation. This process is not only a best practice but also a regulatory requirement in many jurisdictions, emphasizing transparency and the need for explicit communication between the broker and the clients.

Given this context, the other options do not fulfill the legal and ethical standards necessary for broker compensation in an intermediary situation, where informed consent is paramount to protect the interests of all parties involved.