All the pearl buyers are working for each other: they are part of a monopoly to control the market. Basically, the idea is that Kino goes to sell the pearl to one pearl buyer, who low-balls Kino (giving him an offer that is well below what the pearl is worth). Kino then thinks he can go to another buyer to get a better, higher offer for his pearl.
This usually works when there are competeing buyers; however, since all the pearl buyers work together, they all try to get him to sell it for much less than it is worth. The second pearl buyer will offer less than the first for the pearl, while still another buyer will tell Kino it is practically worthless, because it is suposedly too big. They try to make Kino feel like he was lucky to get the previous offer(s). This is because the pearl buyers work together, they want any of their fellow buyers to acquire the pearl for as little as possible in order for the one monopoly to get the most profit possible when they resell it. So every time Kino goes to get a better offer, the buyers will offer him only worse and worse offers. Kino will eventually have to give in, and sell for less than the pearl is worth.