The 500 largest real estate brokerages in the country posted gains in both closed transaction sides and sales volume in 2011 compared to 2010, according to rankings from real estate publishing and communications company Real Trends.But as a group, the 25 brokerages at the top of the list saw a decline in sales volume. Fifteen of the top 25 brokerages on the Real Trends 500 list -- including four of the top five -- saw sales volume decline from 2010 to 2011.  Real Trends ranks the country's 500 largest brokerages each year by transaction sides (in every real estate sale there are two transaction "sides": the buyer's side and the seller's side) and closed sales volume. Transaction sides among the 500 rose to 1.99 million in 2011 from 1.89 million in 2010 -- a 5.2 percent increas...