Willy Loman had a very different set of values than most of society's. He taught Biff that it was ok to steal a football for example, by condoning his behaviour. By saying that his son needed to practice with a regulation ball to get better, he taught Biff that there was nothing wrong with stealing. Consequently, when Biff is in Bill Oliver's office, he steal a fountain pen from his desk for no apparent reason, then runs away.
Willy also taught his son how to close his eyes to reality. Willy is a self proclaimed hot shot salesman who boasts how well liked he is. In reality, however, Willy needs to borrow money from his neighbour Charlie on a regular basis to survive on a weekly basis. We also see how no one knows who he is and this is when he also finally acknowledges the fact that he is a nobody. This type of behavior actually rendered Biff as a disillusioned character as well. He had actually convinced his father, as well as himself, that Bill Oliver knew him - which of course is false.
Willy Loman fails to instill in his son a sense of integrity, honesty, and morality. As a salesman, Willy places an emphasis on being charismatic and well-liked. He teaches his son that having a good smile and positive appearance are all that it takes to be a success. Willy teaches Biff that being well-liked and popular is more important than hard work and dedication. Willy also encourages his son to exaggerate situations and personal accomplishments. Biff learns firsthand from his father how to inflate his ego, which results in his disillusioned perspective. As was mentioned in the previous post, Willy also condones Biff's immoral behavior as a child. Willy does not address Biff's kleptomania and encourages him to cheat on his tests. When Linda mentions that Biff is too rough with the girls his age, Willy insists that nothing is wrong with his son. Later on in the play, Biff attempts to get a loan from his old boss Bill Oliver. Before Biff meets with his old boss, he attempts to date Oliver's secretary so that he can meet with him sooner. During the meeting, Mr. Oliver doesn't remember Biff, and it turns out that he was only a shipping clerk. When Bill Oliver leaves the meeting, Biff is so upset that he steals his fountain pen. Biff's attempt to date the secretary, his disillusioned thoughts about being Oliver's salesman, and his kleptomania can all be traced to Willy's lessons.