Is it illegal for a real estate agent to consult the landlord about selling a house?

Not illegal. It is a normal communication behavior for a real estate agent to consult the landlord whether to sell a house, so it is not illegal for a real estate agent to consult the landlord to sell a house. The real estate intermediary industry is an important part of the real estate industry, which runs through the whole process of economic operation of the real estate industry and provides diversified intermediary services for the production, circulation and consumption of the real estate industry.