What is the nature of the agreement when Broker A agrees to keep details of Seller B's property confidential but will not market it actively?

The nature of the agreement in this scenario is best described as an express unilateral agreement. In this context, an express agreement is one that is clearly stated and explicitly communicated, rather than inferred or implied. Broker A's commitment to keep the details of Seller B's property confidential is a clear and direct expression of intent to maintain confidentiality. This establishes a specific obligation for Broker A.

The unilateral aspect comes into play because it indicates that only one party – Broker A – has made a promise or commitment regarding the confidentiality of the information. Seller B does not need to provide consideration or perform an action in return for Broker A’s promise to maintain confidentiality.

Understanding that express unilateral agreements are characterized by a clear, direct commitment from one party helps highlight the nature of this relationship. Other options, like implied bilateral and conditional agreements, wouldn't accurately describe the specific nature of this promise, as they involve mutual obligations or conditions that aren't present in this case. The verbal expression of the agreement is not sufficient to define its nature accurately compared to the explicit promise made by Broker A regarding confidentiality.