After working for years in real estate, Ishay Grinberg moved from New York to Boston to pursue an idea. That idea came to fruition in 2009 as Rental Beast. Since then, Grinberg has built his rental listing service to cover all 50 states, with offices planted strategically across the country. Expanding seemed logical, according to Grinberg. After the success of the Boston test run came Philadelphia, New York, Atlanta, Miami, Houston, Dallas, Washington D.C. and Chicago. Those are the current places where Rental Beast has set up shop, and there are more on the way thanks to a recent $5 million funding round. The new offices will be located in Los Angeles, Denver and Raleigh, North Carolina. One of the hardest parts, though, is roping in the agents to hear his side of the argument. …