How does injustice exacerbate the plight of the poor?

One of the purposes of the intercalary chapters is to show that not only the Joads are suffering the injustices of much of the capitalist system; thousands of people who must also abandon their homes and ways of life are similarly suffering the slings and arrows of misfortune.
Perhaps the most obvious way that the lowest of the classes are being taken advantage of is in the used car lots that have sprung up all over the sides of highways like intransigent summer weeds. Like weeds, these “dealerships” suck everything they can out of their sources, in this case, the poor, who have no other option but to take what their meager funds can get them.
In a stratified society, those who are just a little bit better off than those below seem to feel a need to mentally place themselves far above the very low. The men who run these impromptu car lots, although no where near even middle class status, nonetheless treat their customers as almost subhuman. Even though each of their customers has very little to spend and must make the best decision for their families, the salesmen care nothing about their circumstances; they care only about how much graft can be gleaned from a particular mark. They watch the potential customers with a mixture of greed and loathing:
Those sons-of-bitches over there ain't buying. Every yard gets 'em. They're lookers. Spend all their time looking. Don't want to buy no cars; take up your time. Don't give a damn for your time. Over there, them two people—no, with the kids. Get 'em in a car. Start 'em at two hundred and work down. They look good for one and a quarter. Get 'em rolling. Get 'em out in a jalopy. Sock it to 'em! They took our time. Owners with rolled-up sleeves. Salesmen, neat, deadly, small intent eyes watching for weaknesses.
Weaknesses, like a husband’s love for his wife, are easy to exploit, as are the man’s concern for his family in general. Further complicating matters is the fact that the majority of these immigrants have never had to deal with people who are aiming to con them. They are confronted with terms they do not know and scare tactics regarding their family’s safety, the veracity of the threats impossible to gauge.
The salesmen also prey on their customer’s social and moral codes. Not far removed from the lower class themselves, they understand how to exploit the desperate travelers’ senses of fairness.
If that doesn’t work, the salesmen change tactics and move to insults. One of the most egregious insults was to be called a “piker,” someone who is stingy and will only place very small bets:
Time and again, Steinbeck shows that community and justice has been replaced by the importance of the individual and injustice. These charlatans will do whatever they have to do to sell a car, and care nothing of the consequences for the customer once the car has been driven off their lot. Underhanded tactics to do whatever it took were not isolated in the time of the Dust Bowl, but commonplace. One trick was to put sawdust in the engine; doing so would briefly quiet any banging noises and also temporarily block any leaks. The “fix,” however, was very temporary and the car salesmen knew it:
"Listen, Jim, I heard that Chevvy's rear end. Sounds like bustin' bottles. Squirt in a couple quarts of sawdust. Put some in the gears, too. We got to move that lemon for thirty-five dollars. Bastard cheated me on that one."
For the displaced families, the choice was take it or leave it.