As sales slow, rental agents are coming on strong. In Manhattan, where about three-fourths of residents are renters, a successful rental agent doing just five deals a month for apartments that rent for $2,000 can earn $108,000 annually on 15 percent fees. Demand for rental apartments has been high, with the vacancy rate expected to finish the year below 2 percent. Rents have been soaring: a studio at a non-doorman, non-luxury building averages $2,151, while a one-bedroom in a similar building averages nearly $3,000.