Earlier quoted context omitted.
He's not world class at selling walls. He's world class at selling Donald Trump.
Unfortunately, selling "walls" (policy) is his actual job. He's like a vacuum cleaner salesman who can always get his foot in the door, but can never manage to sell a unit.
He's like a vacuum salesman who promises to give his customers dishwashers and toasters for free if they buy a vacuum cleaner.
When they buy the vacuum cleaner, it turns out he lied about the other goods but they still ended up with the vacuum cleaner. But what a lot of people really wanted was the dishwasher.
I may have pushed the analogy a little too far...