A represented buyer is a visitor who already has a buyer’s agent. At an open house they are usually there to see the home, not to find an agent, so they are a courtesy contact rather than a prospect you would try to convert.
Separating represented from unrepresented visitors as they sign in is what turns a flat list into a ranked callback queue. The unrepresented visitors, especially those with a near-term timeline, are the ones to call first.
Courtesy contact does not mean no value. A represented buyer who liked the home is feedback you can hand the seller, and their agent is a peer you may be negotiating with next month. The mistake is pitching them: soliciting a buyer who is already under a written agreement is both ineffective and a reputational risk. Note them, thank them, and route your energy to the unrepresented column.