what further strategy does the buyer use to convince Kino that the pearl has little value?

The buyer calls in the other buyers as accomplices.

"Do not blame me," he said quickly. "I am only an appraiser. Ask the others. Go to their offices and show your pearl- or better, let them come here, so that you can see there is no collusion. Boy," he called. And when his servant looked through the rear door, "Boy, go to such a one, and such another one and such a third one. Ask them to step in here and do not tell them why. Just say that I will be pleased to see them."

The pearl buyers argue that the pearl is too large; later the second and third dealers say it is simply a curiosity and nothing more. The highest offer Kino is given is 1500 pesos, a huge sum of money, but a pittance compared to what it is worth.