Most brokers use "Exclusive Right to Sell" listing agreements, which means that all transactions must come through the listing broker. I think your question might be if brokers have listings that are not included in the MLS. Yes, they might. Most are included, but with the seller's permission a broker can withhold it from the MLS. There are usually specific reasons why this is done. That being said, the only way that you, as a buyer, would know about the listing would be if it is advertised or if signage is installed. Hope this helps.