What did the pearl buyers know?
From what I can see, your question refers to events in Chapter 4. In this chapter, Kino decides to take his pearl to the buyers. The author tells us that the pearl buyers know Kino will soon approach them. Each of the buyers contemplates his part in the plan which (he imagines) will bring him incredible riches.
However, the pearl buyers know that they must first work together to pay as little as possible for Kino's pearl. Buying at the lowest cost and selling at a premium price will ensure a fantastic profit for the pearl buyers.
It was supposed that the pearl buyers were individuals acting alone, bidding against one another for the pearls the fishermen brought in.
And once it had been so. But this was a wasteful method, for often, in the excitement of bidding for a fine pearl, too great a price had been paid to the fishermen. This was extravagant and not to be countenanced.
Now there was only one pearl buyer with many hands, and the men who sat in their offices and waited for Kino knew what price they would offer, how high they would bid, and what method each one would use.
The pearl buyers try to convince Kino that his pearl is just a 'monstrosity,' not worthy of its reputation. One buyer offers a thousand pesos; another offers five hundred pesos. However, none of them bargains for Kino's motivation for obtaining a fair price for the pearl. 'My son must have a chance,' Kino vows to Juan Tomas. In the end, Kino rejects the cheating buyers and resolves to go to the capital to ply his luck there.
So, the buyers presume to know the intricacies of human nature; they try to capitalize on the gullibility and fear of the populace but they fail miserably in achieving their goals through Kino.