Salesmanship is the act and ability to convince potential customers to purchase items. The better the salesperson, the more salesmanship he or she will display. The key to salesmanship is not just talent, but also discipline. It is then discipline to do more than just sell. True salespersons also have to tolerate difficult customers, create a habit of putting themselves out there, they move their brand through the channels, and work "the streets" to promote their product. In other words, salesmanship is a whole package of hard work that brings on,both, street smarts and experience.
In the case of Willy Loman, it is arguable that his own salesmanship may be in question, precisely because we see neither discipline nor the motivation to build it among the Loman males. It because of this lack discipline and hard work that Willy also lacks success. Willy seems to detour from the focal goals that all salespeople stick to and, instead, wants to reap the benefits of hard work when it is others who have done it.
This said, the Lomans are better at jumping at quick opportunities than at devoting time and dedication toward building the goals that come with salesmanship. Rather than lure people in, and work their way up, the Lomans take what they can from every one else: Willy takes money from Charley, but refuses to accept a job from him. Biff steals everything under the sun as a way to compensate for the lack of discipline to keep at job without ruining something. Happy lies to people, especially women, luring them to trust him, only to betray them. This is also another example of crass discipline and respect. What else is left for the Lomans, when they really don't have what it takes? Chaos.
Hence, this is the relationship between stealing and salesmanship: The difference between the two is that adhering to salesmanship requires discipline. Moving away from such discipline leads to chaos. Chaos is a common denominator in the Lomans, especially with regards to Biff.
In the play, stealing occurs in different ways:
1. Biff steals Bill Oliver's pen when the latter leaves him waiting for what would have been a failed attempt at securing a business loan from Oliver.
2. When he is in high school, Biff steals a regulation football from his coach, presumably because he wanted to practice with it the touchdown that he would score for Willy, to this, Willy commends him by saying:
"Sure, he's gotta practice with a regulation ball, doesn't he? Coach'll probably congratulate you on the initiative."
3. Younger still, Biff and Happy steal lumber for one of Willy's project. Willy completely disregards the act and instead tells Charley how proud he is that his children brought it in.
4. Biff also stole a previous time from Bill Oliver. He stole a crate full of basketballs, and this is the primary reason why he quit: He did not know if Oliver knew about it, and so Biff left before he could find out.
Therefore, the stealing makes up for the salesmanship in the world of the Lomans. Whatever can be obtained quickly and effortlessly is seen as a "gain," whereas a true salesperson would have used discipline to establish and attain goals.