It’s usually a relief to see a stock rebound after a big sell-off. But not all rebounds indicate strength. In cases where a stock has made a big advance, a high-volume sell-off followed by a weak rebound in thin turnover is a signal that it’s time to sell and lock in your gains. In June 2013, Lululemon Athletica (LULU) made a steep decline after a huge run that began in the summer of 2009.