From the introduction:
Goblins are not known for being especially intelligent in the musty, wizardy-sense of the term. However, they are crafty and can be quite shrewd. Every once in a while, when the conditions are just right, they get together to hawk their wares. While there is a certain degree of goblin-to-goblin trade occurring at such markets, these events are in the main intended for outsiders. A goblin market will spring up in a location that is far from the eyes of the Man, as goblins tend to deal in items that are generally considered unsavory, dangerous, or too cryptic to be entirely trusted. The typical customer is one who is not averse to the magical, one who is seeking something outside the scope of what is offered in the stalls of the town’s commons.