Two years after they were taken over by the federal government, Fannie Mae and Freddie Mac face a new challenge: The mortgage-finance giants are becoming two of the nation's largest home sellers at a time when the housing market shows new signs of softening. Realtor Brett Barry, standing Tuesday in the kitchen of a ranch-style home in suburban Phoenix owned by Fannie Mae, says of a recent price reduction to $200,000 from $265,000: 'They're definitely pushing the envelope on price. But they're doing it at the wrong time.' Fannie and Freddie have already taken back nearly as many homes in the first half of the year as they did all of last year. They owned more than 191,000 homes at the end of June, double the year-earlier total. That number will grow because they are taking back homes faster than they sell them...