An agency by estoppel, which is an implied agency, was created since Lev, the principal, did not stop Francois, the agent, from representing Lev’s interests. An implied agency is formed based on the words or actions of the agent and principal that evidences an intent to form the agency. An express agency is formed through an oral or written agreement. An agency does not necessarily need to be created through a written agreement. A listing agreement is an example of an express agency.