Cameras follow four bible salesman and record their interactions among themselves and customers. Typically they interact with housewives, but sometimes others enter the picture. In a bizarre moment a husband starts playing loud music (Muzak versions of Beatle songs) on a gigantic stereo as the salesman performs his pitch. The camera captures every nuance as they employ every means necessary to sell their bibles.
Early on in Salesman, their boss, a humorless motivational speaker, delivers a soul sucking speech basically saying if you cannot make the sales you are a failure as a person. Meanwhile other salesman stand up and deliver "motivational" speeches about their prowess with sales, each trying to top the other. That is their world.
The film pays particular attention to "Badger", an aging salesman in a serious slump. As he observes his younger colleagues achieve success, he grows increasingly cynical. So he starts to use questionable methods such as putting customers on guilt trips, shaming them, even going for their sympathy. Despite his pettiness, we oddly identify with him and root for him to get the sale.
Unlike modern documentaries, Salesman's not an op-ed piece. The camera simply reveals a reality. There's no talking heads, animated sequences, or stock footage from old movies. What you see on screen is what you get.