Matty found it strange that none of the people who came to the Trade Mart seemed to have anything to trade.
Seer had never attended Trade Mart, and was proud of it. This made Matty curious, because Trade Mart was an old tradition and he wanted to know how it worked. So he decided to try it.
The Trade Mart is not what Matty expected.
There was an intentness to everyone, an odd seriousness, and a sense of worry—unusual in the village—pervaded the atmosphere. (p. 56)
Matty realizes that the place is strange and this is why Seer did not want him to come. When Mentor asks him if he is participating, he says he has nothing to trade. Mentor assures him that everyone has something.
When Matty returns, he explains to Seer how the Trade Mart worked. People yelled out what they were trading, and whispered what they were trading for. He realizes that people are trading for intangible things, and Mentor’s birthmark is gone.