Well he's not as smart as he thinks he is. He scolded someone for a grammatical error that he was wrong about. Buyer can be both plural and singular (it was used incorrectly by the responder, however). An example of that would be when you refer to a company or business as your "buyer". The company/corporation is a singular unit made up of many people so you would be right to use the phrase "I sell manufacturing equipment to Pepsico, they are my buyer".