In the first chapter of the book, we see why Barney Northrup can be described in this way. First, we see that he gets results when he tries to sell things. He was able to sell all of the apartments in Sunset Towers in just one day. Second, we can see that he is pretty slick in his approach to selling. We see that he tells every single one of the people that he is selling to that there is only one apartment left. He does this to make them feel like they really need to buy the apartment right away rather than going away to think about it. These things show us that Barney Northrup (this is, of course, an alias) is a good salesman.