You expect them to know about payloads? Consider this:
In 2006 I went to purchase a diesel tow vehicle. I had owned a fleet of diesel equipped vehicles back in the 80's. I told the salesman, who was touted as a "Truck Specialist," of my 20 year experience. I was out of date with diesels and asked:"Do today's diesels still have a "Wait to start" light on the dash?"
His reply:"Oh no. They did away with those years ago. You can just start them right up." During the test drive I saw the "wait" light illuminate on the dash!
Having some knowledge allowed me to realize that this "truck specialist" knew absolutely nothing about trucks. It was fun though, I had lots of fun throwing questions at him that he did not know the answer to, during the negotiations. Mostly, his answers were lies.
I would never assume that a salesman has any product knowledge. They were trained how to sell, not about the product.