NEW YORK ( TheStreet) -- Rising home prices have made the business of "flipping" houses more profitable. According to a report from RealtyTrac, there were 136,184 single-family home flips -- where a home is purchased and subsequently sold within 6 months -- in the first half of 2013, up 19% from a year ago and up 74% from the first half of 2011. Real estate investors made an average gross profit of $18,391 on single family home flips in the first half of the year, a return of 9% on the purchase price. In the first half of 2012, investors made a gross profit of $5,321. In the first half of 2011, when home prices were still falling, flippers made an average loss of $13,206. Real estate investors have dominated the housing recovery, with many first-time home buyers and trade-up buyers lacking access to credit or the equity to buy a home. Investors meanwhile have plowed cash into foreclosed homes and fixed them up in the hope of selling them at a profit as home prices recover. Until recently, the recovery in housing has been slow, resulting in many investors holding on to properties longer than six months and choosing to rent them out instead. But with home prices recovering more strongly now and most parts of the country experiencing a shortage of inventory, the number of flips is once again climbing. Still, in 32 out of the 100 markets analyzed by RealtyTrac, including "perennial flipping hot spots like Las Vegas, Phoenix, Southern California and Atlanta," flipping is on the decline. "While flipping continues to be profitable in most markets, particularly those where the home price recovery is still nascent and a recent rebound in foreclosure activity allows investors to find distressed inventory at a discount, home flipping is tapering off in markets where fewer of those distressed bargains are available," said Daren Blomquist, vice president at RealtyTrac in a release. Strong rental demand may be another factor that is limiting the number of flips, as rental yields are still attractive in many parts of the country.