LAKE FOREST, Calif.—Jeff Pintar had buyer's remorse as he purchased 12 foreclosed homes in five Southern California counties on a single day. His regret: that he didn't buy more homes a year earlier.

"Things have turned around faster than anyone anticipated," said Mr. Pintar, who first began buying properties here four years ago and now owns or manages 1,700 homes, which he rents out for between $1,000 and $3,800 a month. Here in Orange County, nearly every home listed for less than $400,000 "is being pursued...