Biff has a history of stealing, first as a teenager and later as an adult. While still in school, Biff stole a carton of basketballs. Later he "borrowed" a football from the school locker room, without the coach's knowledge or permission. Biff also got through school by stealing grades; he cheated by getting answers from Bernard who could not refuse him.
When Willy wanted to rebuild the front porch of their house, Biff and Happy stole lumber. Charley warned that if the boys stole any more lumber, "the watchman'll put the cops on them!" Willy, however, was pleased with his sons:
You shoulda seen the lumber they brought home last week. At least a dozen six-by-tens worth all kinds of money.
After leaving school, Biff's thievery continued. He stole a suit and went to jail for three months in Kansas City. He tells his father at the conclusion of the play this hard fact: "I stole myself out of every good job since high school!" Biff's final act of theft occurs toward the play's conclusion when he sneaks into Bill Oliver's office and steals his fountain pen. Biff's habit of stealing and his inability to hold a job underscores the corruption of his character and Willy's failures as a father.