These are regulated by the Department of Consumer Protection.  See CGS Chapter 734a,  Section 42-103c.  Defined as “Any person who, in exchange for a fee, permits a customer to inspect or otherwise utilize a listing of apartments for rent.”

No person may engage in business as an apartment listing service without first obtaining a certificate of registration from the Commissioner of Consumer Protection, except that a person holding a valid real estate brokers license in this state shall not be required to obtain a certificate.