Sales of previously owned, single-family homes increased 2.5 percent in May, according to new numbers from the National Association of Realtors. The gains follow two months of declines and are evidence that home buyers are responding to improved affordability conditions. Lawrence Yun, NAR’s chief economist, says, purchase power is up. “The purchasing power to buy a home has been bolstered by falling mortgage rates, and buyers are responding,” he said. But though favorable buying conditions are good for potential house hunters, increasing demand comes with its own set of challenges. For example, the typical property was on the market just 26 days in May. Additionally, 53 percent of homes sold were on the market for less than a month. In other words, good homes are selling fast. That means, home buyers looking for a house this summer need to prepare for competition. Before heading out to look at houses, buyers should make sure their finances are in order, they’re prequalified to borrow, and that they’re ready to make an offer when they find a house that fits their life and budget.