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.

The minimum bid, as set by a unit of Citigroup Inc., which had a $1.3 million mortgage on the home, was $379,900. After several minutes of bidding among investors and their representatives, some wearing shorts and flip-flops, Mr. Mirmelli won the home for $486,300. A week later, he agreed to sell it for $690,000 to a woman who moved in this month.

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Flippers swoop in at public auctions of foreclosed homes, known as trustee or sheriff sales. In many states, the lender sets the minimum bid, and takes possession of the property only if no one bids more. In the past, the minimum generally was about equal to the mortgage balance due. But in today's market, in which many home values have dropped far below the loan balance, lenders wouldn't attract investors if they set the minimum at that level.

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Sean O'Toole, chief executive officer of ForeclosureRadar.com, a research firm, estimates that in November about 21% of homes sold in trustee sales in California went to investors rather than to a foreclosing lender, up from 6% a year earlier. The trend is similar in some other areas with high foreclosure rates, including Phoenix and Miami.

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The risk for banks is that if they set the minimum bid too low, the home might end up selling for much less than they could reap if they took ownership of it and sold it themselves. But with some 7.5 million U.S. households behind on their mortgage payments or in foreclosure, many lenders are overwhelmed. They're negotiating with distressed borrowers and figuring out how to sell the growing supply of foreclosed homes.

"The banks are so screwed up," says Mr. Mirmelli, the Phoenix investor, that they don't always have a clear idea of the value of the property they are foreclosing on.