The industry couldn't function without the armies of agents who help buyers and sellers reach mutually agreeable terms on those four-bedroom, center-hall colonials, and who generally collect hefty 6 percent commissions for their trouble. But a recent study by two University of Chicago economists suggests that home sellers should regard agents with some caution. The study does not suggest that agents are inherently untrustworthy. Rather, it says, the housing market remains inefficient, and the incentives for agents to maximize profits for their clients aren't powerful enough.