Be very careful, if you are not an owner in the property. Otherwise, you it is highly likely that you need a license to practice real estate.
By your question it appears that you are connected with the residential real estate market, in that you use the term "Realtor", which largely is made up of residential agents, who belong to the National Association of Realtors".
Most leasing, other than the leasing of a single family home, or perhaps units of four or less is done by commercially trained and experienced real estate brokers, or agents working under a broker.
It would be my recommendation before doing any leasing other than as an owner, that you associate with a real estate office, as a licensed real estate agent, where you can receive excellent training and guidance in commercial real estate.
Onward and Upward....
Oct 30, 2012