When the Federal Reserve vowed last week to buy up $40 billion worth of home loans each month for the indefinite future, the hope was to finally stimulate a mortgage market on life support for nearly five years. That may yet happen. The National Association of Realtors on Wednesday reported that sales of pre-owned homes in the U.S. jumped 7.8 percent in August to a seasonally adjusted annualized rate of 4.82 million units, the highest in more than two years. The median home price was up nearly 10 percent from a year ago. But obtaining a loan, especially for low-income borrowers or borrowers with less-than-stellar credit, remains difficult or impossible. Even many buyers with exceptional credit remain on the sidelines, fearful of another downturn or frustrated by their experience dealing with banks that don’t seem particularly interested in lending.