Salespeople can get away with being uniformed when their customers are less informed than they are. However, once the customer knows a lot about something, then the salesperson becomes unnecessary and is reduced to "taking orders" only. And this makes it difficult to sustain the higher markup in retail stores.
The two best examples of this are probably in computer sales and digital camera sales. Initially, their customers knew very little, so they depended on sales people to help them make a buying decision. Thirty years ago, retail stores flourished and salespeople earned fat commissions whether or not they knew anything about the products they sold.
But once the customers learned more about these products, many knew more about them than the people selling them. And when that happens, the product becomes a commodity, and the mail order or online retailer becomes a much cheaper source. The customer knows exactly what he wants, and the only questions left is "how much does it cost" and "is it genuine?"
So most brick and mortar retail shops for computers and digital cameras have closed, and online retailers have flourished. Not many customers need a salesman to help them select what they want. Once the customer knows what they want, then it just becomes a question of price, availability and warranty.
You pay more at the authorized watch dealer, not because they are helping you make an informed decision, but because it is often the only way to get something in high demand. Plus, you feel there is a greater chance that the item is genuine, and not a fake.