Four years after the collapse of the U.S. housing bubble, flipping homes is back in fashion.

Jon Mirmelli, a Phoenix real-estate investor, learned late in the morning of Sept. 28 that a never-occupied custom house on the northern fringes of this Phoenix suburb was going up for auction around noon the same day. The six-bedroom home, built on a three-acre desert plot, has a kitchen with two dishwashers, four ovens, "antibacterial" copper sinks, and a master "spa" bathroom with space for a flat-screen TV visible from the tub. Flipping Foreclosures