Is Commission received by a dealer from the buyer and seller of a house included/excluded in National Income?

Yes,

It will be included in the national income as it is the income of the dealer for his productive services.

dealer provides its services in this whole transaction for sale make happen. He arranges the meetings of the various parties. It helps in negotiation.

It adds to the flow of goods and services in the economy.

Thus, it is included in the National Income of a country.