The buyer basically represents capitalism in its most corrupted, least functional form. Think of him as a Monopoly Man with devil horns.
A quick EconLite lesson: the whole point of a free market economy is that competition is healthy. In this case, the buyers’ competing for Kino’s pearl should assure him the best possible price for his goods. Sounds good, right? Sure... until (dum dum dummm) human evil gets involved.
Basically, the buyers are colluding. They're rigging the sale and offering way, way less than the pearl is worth. They've corrupted capitalism with their greed... in much the same way the pearl is corrupted by jealousy and Kino’s dreams are corrupted by the hatred of others. And (like the pearl and Kino’s dreams) this doesn’t necessarily mean that capitalism is evil; it means the men have made it evil through their actions:
[...] Kino [...] felt the evil coagulating about him, and he was helpless to protect himself. He heard in his ears the evil music. And on the black velvet the great pearl glistened, so that the dealer could not keep his eyes from it. (4.42)
If Steinbeck is critiquing capitalism—and that is very much subject to debate—he probably is not critiquing it per se. It's more likely that he's suggesting that a system requiring honesty and fairness might not work in our incredibly imperfect world.