If you are watching for signs of improvement in the housing market so that you can finally put your home up for sale, you might be shooting yourself in the foot. In May, Zillow.com, an online real-estate company, reported that about 25 percent of Midwest homeowners say they are at least somewhat likely to list their residences if they spot positive signs in the market. Should that happen, warns Stan Humphries, Zillow’s chief economist, it could create a glut of homes and possibly extend the market’s slowdown. “It’s a classic collective-action problem,” he says. “What’s rational for the individual becomes irrational for the collective body. Individuals try to maximize their economics by liquidating their homes, but when a lot of people make that decision, it becomes bad for everyone.”