For the first nine months of this year, U.S. fund managers could be counted on to tout the same line: the stock market was getting so expensive they could do nothing but sit on their hands. Yet the nearly 8 percent decline in the S&P 500 over the last two weeks has some top-performing portfolio managers buying for the first time in months.

"We're finally getting busy again," said Scott Moore, the lead portfolio manager of the Buffalo Dividend Focused fund, the top-performing large-cap fund for the year to date, according to Morningstar data. Over the last two weeks, Moore has used the more than 10 percent of his portfolio held in cash that he built up by trimming positions to add to companies ranging from energy stocks to healthcare companies to master limited partnerships, he said.

Even on a day when major indexes are down sharply, Moore has company. Several fund managers contacted by Reuters said that the sell-off in the U.S. stock market, punctuated by a more than 400 point drop, or nearly 3 percent, in the Dow Jones Industrial Average in midday trading Wednesday, is prompting them to buy. They do not see the sell-off as the start of the first bear market since the end of the financial crisis in 2009, fund managers said.

The Standard & Poor's 500 index last had a drop of 10 percent or more, a decline known as a correction in the jargon of Wall Street, in 2011. Since that time, the index has rallied more than 60 percent, pushing the price to earnings ratio above 18, toward the high end of its historical average. Some popular growth stocks like Netflix Inc have seen their price to earnings ratios climb above 131 over that same time, a sign that some analysts took as a sign that the stock market was becoming frothy.

Fund managers say that the market had gotten ahead of itself and stocks now trade at something closer to attractive values.

Michael McGarr, a co-manager of the Becker Equity Value fund, said that he has been drawing down on his cash position built up from trimming his portfolio after the S&P 500 crossed 2,000 in mid-September to buy shares in companies including agricultural equipment marker ACGO Corp, rail company CSX Corp, and pharmaceutical firm Pfizer Inc. The S&P 500 fell 2.3 percent to 1,834 in mid-day trading Wednesday.

"This sell-off is resolving some of the valuation concerns that we had earlier this summer, and it looks like an opportunity to pick up some new ideas that could be decent long-term holdings," McGarr said.

Not every fund manager is rushing in, of course. Tom Sudyka who co-manages the LK Balanced fund, said that his firm is waiting until the S&P 500 falls 10 percent from its Sept 19 record high of 2,019 to start buying. High prices have led Sudyka to add two stocks to his portfolio this year, he said, down from 4 or more in a typical year.

Bill Smead, the portfolio manager of the Smead Value Fund, said that he has been buying Aflac Inc and eBay Inc during the last week as their prices fell. He also sees a silver lining: the drop in the 10-year Treasury yield below 2 percent and plummeting gas prices in the U.S. should help younger people lock in low-priced mortgages and have more spending money, potentially helping the U.S. economy expand.

"Bull market corrections are historically fast and violent and take your stomach away. I don't know if this is the bottom or not, but we're not thinking this is going to be a rerun of 2008," Smead said.