Elayne’s “joke” about the Aes Sedai

In chapter 5 Elayne thinks about a joke she once heard about a rock being sold back and forth between a Domani and one of the Sea Folk and them always making a profit. Then an Aes Sedai comes along and the Domani convinces her to buy the rock for double what the Domani paid. After which the Atha’an Miere convinces the Aes Sedai to pay twice that again. The passage ends with “Only a joke, but it showed what people believed.”

I am so confused, I don’t get the joke and I also don’t understand what it’s trying to imply people believe about the Aes Sedai?