Much like the newsreel style of John Dos Passos in U.S.A. Trilogy, Steinbeck's intercalary Chapter 7 portrays rhetorically the cold exploitation of the used car salesmen who sell vehicles to the dispossessed sharecroppers like the Joads.  Their staccato speech and the staccato sentences that switch from topic to topic connote the bewilderment and confusion associated with the used car lots as well as the disfranchised life of the sharecropper headed to California:

Hot sun on rusted metal.  Oil on the ground.  People are wandering in, bewildered, needing a car.

In addition, the used car salesmen are metaphors for the many people that the Joads and others will meet who will be cruel or take advantage of them as they travel to California:

All right , Joe.  You soften 'em up an' shoot 'em in here.  I'll close 'em, I'll deal 'em or I'll kill 'em.  Don't send in no bums.  I want deals.

Didn't you never hear about carrying charges and insurace?  That just boosts her a little.  You'll get her all paid up in four-five months.  Sign your name right here.  We'll take care of ever'thing.

In the juxtaposition of the agrarian life with mechanical world, the mention of the men trying to trade their mules for a truck, Steinbeck suggests the oxymoronic life into which the Joads and others are driving.