In this scene, Willy has an imaginary encounter with his dead brother Ben. No matter how real the details of Ben's story, Ben represents to Willy the material success that he longs for but that has always eluded him.
Willy imagines an encounter between Ben and Biff, in which Willy encourages Biff and Ben to fight. He wants to show Ben that he has brought his sons up ruggedly: "rugged, well-liked, all around," is how he puts it. He hopes to impress Ben.
Ben ends up tripping Biff and stands over him with the tip of his umbrella held above his eye. Ben has suddenly turned into a stranger and he tells Biff, "never fight fair with a stranger."
Multiple things are wrong with this idea. First, "never fight fair" contradicts Willy's bedrock notion that you get ahead by being likable. It thus contradicts what he is purporting to show Ben, which is that he is raising Biff to be "well-liked" as well as rugged. Willy, in an inchoate way, is perhaps recognizing for a moment that his own cherished ideas of success are "strange" to his brother. Further, Biff is no longer interested in his father's delusions of what constitutes success and his family's role in it. Ben's imaginary advice is unlikely to have any weight with him. Finally, the play calls into question the very notion of success that Willy clings to and admires in Ben. Willy, as usual, is worried about the wrong things, such as impressing his brother or getting ahead materially, going for the big financial kill.
I suspect that this provides an alternative to Willie's theory of life. Willie believes that being well liked, personality, is the key to success in life. Biff sort of play fights with Ben (depending on the production you see); when he finishes, Ben drops him and explains, by example, that it's not "niceness" that gets you ahead in this world, but the exercise of "power."
If the business world is a "jungle," then Ben presents the case that the exercise of power, even ruthlesness perhaps, is the way to get ahead; it's a play on the old saying that "nice guys finish last."
I'm not sure there's anything wrong with it ... unless you're a Willie.