Biff's stealing is not a weakness; it's a symptom. Early in the play, Biff shows his father a football he has "borrowed" from school:
BIFF: Did you see the new football I got?
WILLY (examining the ball): Where’d you get a new ball?
BIFF: The coach told me to practice my passing.
WILLY: That so? And he gave you the ball, heh? BIFF: Well, I
borrowed it from the locker room. (He laughs confidentially.)
WILLY (laughing with him at the theft): I want you to return that.
HAPPY: I told you he wouldn’t like it!
BIFF (angrily): Well, I’m bringing it back!
WILLY (stopping the incipient argument, to Happy): Sure, he’s
gotta practice with a regulation ball, doesn’t he? (To Biff.)
Coach’ll probably congratulate you on your initiative!
This is Willy's fault; in no way should he be encouraging his son to steal anything. Stealing is a sign, a symptom of weakness in character, of want to have sonething that you didn't earn, be something that you're not.
And this isn't the first time Willy abets stealing. In one of Willy's many flasbacks. Willy is bragging about his boys to his brother Ben:
CHARLEY: Listen, if they steal any more from that building the
watchman’ll put the cops on them!
LINDA (to Willy): Don’t let Biff...
(Ben laughs lustily.)
WILLY: You shoulda seen the lumber they brought home last
week. At least a dozen six-by-tens worth all kinds a money.
CHARLEY: Listen, if that watchman...
WILLY: I gave them hell, understand. But I got a couple of fearless
CHARLEY: Willy, the jails are full of fearless characters.
This is more than giving his sons mixed messages; Willy is encouraging and praising theft.
So, stealing has been part of Biff's false, propped up life from the early days. But when Biff commits his last theft, the fountain pen from Bill Oliver's desk, he finally come to terms with what he has been doing all along (Act 3):
BIFF:... I ran down eleven flights with a pen in my hand today. And suddenly I stopped, you hear me? And in the middle of that office building, do you hear this? I stopped in the middle of that building and I saw — the sky. I saw the things that I love in this world. The work and the food and time to sit and smoke. And I looked at the pen and said to myself, what the hell am I grabbing this for? Why am I trying to become what I don’t want to be?
You see what I mean by stealing being a symptom?
Biff's weakness throughout the play is his inability to see things as they are, to see himself through his own lens and not through Willy's. Granted, he probably is closer to reality than any of the other Lomans, but he can get sucked into their illusions pretty easily.
For instance, in Act 1, Biff and Happy discuss careers and women. Biff would love for the two of them to go out West and buy a ranch: "Men built like we are should be working out in the open." In this conversation, it is obvious that Biff thinks he is a cut above the rest, that he and Happy are true men, "that they'd be known all over the counties" should they become partners.
In many ways, Biff believes what Willy has taught him about himself. Biff believes that he does not have to play by the rules that others play by. He can steal a football if he needs to practice more, he can steal lumber from a construction site, he can play rough with the girls, he doesn't have to study in school. When Biff faces a math teacher who won't be persuaded into passing him simply because of who he is, Biff becomes derailed. This incident is followed by Biff seeing his father with a mistress. This incident was particularly traumatic because Biff's image of his father becomes tarnished, and Biff is too shocked, too disappointed, but also too proud to go to summer school and get his high school diploma. He goes West to make his fortune--and fails. Deep down, Biff blames his father for his failure, not himself.
As an adult, Biff sees the weakness in his father. He knows his father is talking to himself, living in the past, barely making a living, suicidal, but when he tries to confront his father, he becomes diverted by Happy and Willy's pipe dream: The Loman Brothers Line of sporting goods. When the other Lomans begin getting excited about such a prospect, Biff is sucked into believing that such an enterprise could be successful, that he and Happy could strike it rich. He even makes plans to see Oliver, a man he worked for in high school, whom had stolen from before, and ask for a substantial loan. Willy calls Biff a young Hercules, and Biff still believes that he can do the improbable--all he needs is a break.
At the end of the play, Biff comes to face the fact that he, like his father, is a loser. He has not the slightest chance of getting a loan from a man who hardly remembers him. He tells Willy, "I am a dime dozen, and so are you!" He goes on to say
I am not a leader of men, Willy, and neither are you. . . I'm not bringing home any prizes any more, and you're going to stop waiting for me to bring them home!
Biff tells his father to burn his "phony dream," and that he is leaving in the morning. Biff is severing his ties from his father, and he is going to live his life away from the shadow of his father's image and expectations of him. In the requiem, he tells Happy, "I know who I am, kid."
In the play The Death of a Salesman Biff Lowman goes to see his high school friend to discuss the possibility of his former friend lending him money for a financial venture. Biff had been jailed for stealing before. However, at the office he is left waiting all day for the man to see him. As time goes on he comes to realize that the man is blowing him off and had never intended to meet with him. Biff picks up his pen and steals it.
Biff starts out as a punk in the beginning of the play. He is a dreamer who always has a scheme going that never pans out. He is still dreaming up a scheme until he comes to terms with the fact that he has no ability to be successful as an entrepreneur. After he leaves the office where he was left waiting all day and not given the time of day, Biff comes to realize that his life has been a lie.
Biff decides that he needs to start being honest about things in his life. He tells his brother that they have to stop lying about who they are. He decides that he needs to get a job and stop pretending.