The people in the market saw that this was a very special fish (because it was still alive although out of water), and so they all wanted to buy it because they expected that eating it could bring immortality.
But Shancai was able to buy the fish. The other customers were at first angry that the boy took the fish from them, but then they felt sorry after Guanyin personally intervened.
Guanyin then returned the fish-boy to the sea. In gratitude for receiving his son back, the dragon king gave Guanyin a pearl that magically shone light