Who Can Be Trusted in Glengarry Glen Ross?

The salesmen in David Mamet's acclaimed play Glengarry Glen Ross are bilking the public by selling them land that could be completely worthless or worth only a fraction of what they paid. It is natural that such unscrupulous businessmen should behave the same way towards their colleagues.
Richard Roma seems to be above such chicanery. When he has his argument with Dave Moss he tells him, in effect, that they should all be pals and allies. It looks as if Roma and George Aaronow are at least honest with their co-workers. The others demonstrate by their behavior within the office that they know this is a dog-eat-dog world and they are going to be the dog that eats rather than the dog that gets eaten.
Dave Moss tries to exploit George Aaronow by involving him in a plot to burglarize the office and steal the coveted Glengarry leads. We later learn that Moss exploited Shelly Levene's financial distress by getting him to commit the crime after Moss apparently realized that Aaronow would not cooperate or would not make an effective burglar.
John Williamson, the office manager, intends to exploit Shelly Levene by selling him "the good leads" for fifty dollars apiece plus twenty percent of any commissions. Later Williamson will trick Shelly into confessing that he burglarized the office by pretending that he won't tell if Shelly confesses. Then when Shelly admits the truth, Williamson goes directly to Baylen the investigating cop.
Levene betrays Dave Moss and also betrays Jerry Graff, who could go to prison for knowingly receiving stolen property. George Aaronow so far has not betrayed Dave Moss for suggesting the burglary to him earlier, but he could get around to it if Moss brought Aaronow's name into it, as he had threatened to do.
Mitch and Murray, of course, are exploiting the entire sales staff and trying to exploit the general public. Moss is referring to them when he says: "I'll go in and rob everyone blind and go to Argentina cause nobody ever thought of this before."
Roma is trying to exploit James Lingk and his wife by selling them subdivided plots of land at exorbitant prices. When Lingk comes to the office Roma lies to him that his check has not yet been cashed, so there is no urgency about cancelling the contract.
Levene is a pathetic character, but he tries to exploit Williamson by faking a burglary and stealing all the Glengarry leads. Levene is also injuring Mitch and Murray as well as his fellow salesmen Roma and Aaronow.
Shelly Levene also tries to help Roma cheat Lingk by posing as a big executive with American Express and lying outrageously about all the land he has supposedly bought from Roma. Levene is a somewhat sympathetic character, but he is the biggest liar of them all because he is the best.
At the very end of the play (but not the film version) Roma reveals his superlative deceitfulness when he tells Williamson he wants to keep his own leads but share Shelly's with him fifty-fifty. Not knowing that Shelly is out of the real estate business and probably on his way to prison, Roma tells Williamson: "Well I'm going to worry about it, and so are you, so shut up and listen. (Pause) I GET HIS ACTION. My stuff is mine, whatever he gets for himself, I'm taking half. You put me in with him." We see that Roma, who has just been flattering and cajoling Shelly, calling him Levene the Machine, is the greediest, craftiest, and most ruthless one of all. He plans to take half of Shelly's commissions without giving him a nickel of his own commissions. Some partnership!
The salesmen in Glengarry Glen Ross have become so competitive and so hardened by their profession that they can turn against each other without pity. Such salesmen were called "land sharks" at the time the play was produced, and like sharks they are capable to turning on a bleeding member of their own species and devouring it to slake their insatiable hunger.