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BIFF: I guess so. I know something about it and–
WILLY: He knows something about it! You know sporting goods better than Spalding for God’s sake! (Act 1)
Willy is deceiving himself yet again. You're probably not surprised. Here, he is attempting to convince Biff that, because Biff was a high school football star, he knows more about sporting goods than professionals in the industry. Umm, not so much, Willy.
BIFF: I guess so. I know something about it and—
WILLY: He knows something about it! You know sporting goods better than Spalding for God’s sake! How much is he giving you?
BIFF: I don’t know, I didn’t even see him yet, but—
WILLY: Then what’re you talkin’ about?
BIFF [getting angry]: Well, all I said was I’m gonna see him, that’s all! (Act 1)
Willy deceives himself into believing that Biff has already sealed a deal that his son has not yet even acted on. Willy's self-deception eventually drives Biff to lash out at his father.
BIFF: I am not a leader of men, Willy, and neither are you. You were never anything but a hard-working drummer who landed in the ash can like the rest of them! I’m one-dollar an hour, Willy! I tried seven states and couldn’t raise it. A buck an hour! Do you gather my meaning? I’m not bringing home any prizes anymore and you’re going to stop waiting for me to bring them home! (Act 2)
Biff insists he be left alone to live his life. He's begging his father to allow him to measure his personal success in his own way. Biff no longer wants any part of Willy's delusions.