Real estate agents are increasingly pitching sellers on their version of secretive dark pool trading: Skip the multiple listing service (MLS) and let the agent market the home privately. Or it can mean posting a Coming Soon sign on a property before listing it on the MLS. Then there’s the growing number of private agent clubs, closed networks of agents who deal among themselves, often using “pocket listings,” or homes they have yet to — and may never — post to the MLS. No national data exist on the percentage of residential transactions happening off the MLS.