Don't worry. McBean's machines aren't actually symbolic of a Ponzi scheme. We just liked the almost-rhyme. So what do they symbolize? We're going to say the mechanics of capitalism. Yep, take a seat—things get bumpy from here on out.
McBean's first machine is the Star-On Machine. He "[puts] together [this] peculiar machine" (Sneetches.35) to help the Plain-Belly Sneetches get them stars upon thar bellies. It seems like he's just there to help. As for the three dollar charge, well, that's just a thing.
Then McBean builds a Star-Off Machine. This contraption helps the original Star-Belly Sneetches become the "best Sneetches on beaches" once again (Sneetches.56). Ten dollars must be paid up front—naturally.
And so, through McBean's machines, we get a nice little view of how Seuss sees capitalism. The Sneetches think they have a problem, so McBean constructs a machine to fix said problem. The Sneetches then keep paying (over and over again) to fix the problem—which may not have been there to begin with if not for McBean. Once caught between the machines, it's easy to get lost in the chaos of the Sneetches. Not a great outcome.
Seuss might warn you to remember the McBean machines the next time you think you need a knife that can cut through a shoe.
Don't forget to mosey on over to the "Characters" section for more about McBean's machines.