CINCINNATI, Ohio (Reuters) - When Keith Gersin saw the perfect four-bedroom house in southern Ohio four years ago, he jumped to buy it before anyone else could snap it up. When he finally sold it last month, it went for $30,000 less than he had hoped -- and that after seven months on the market. In retrospect, the 41-year-old physician admits he overestimated the U.S. housing market, which has begun cooling after five years of record-breaking sales and double-digit price appreciation. "I was naive," said Gersin, who sold his Cincinnati-area home in May to move to North Carolina with his...