How do people of Umuike develop their market?

Essentially, the people of Umuike develop their market through the use of magic.
In Chapter 12, Obierika is preparing for his daughter's wedding. When Okonkwo arrives at Obierika's compound, everyone is hard at work preparing for the wedding feast. Two goats have already been slaughtered for the soup. Meanwhile, a huge goat is tethered to a peg near the compound wall. Obierika intends to present this goat to his in-laws at the wedding feast. The goat was supposedly purchased at the Umuike market, which is said to be a wonderful place.
According to Obierika, magic ("great medicine") contributed to the popularity of the Umuike market. Essentially, the people of Umuike used supernatural means to ensure that their market would dwarf others in the region. Obierika maintains that this magic can be seen in the shape of an old woman with a fan every market day. It is this old woman who beckons everyone from neighboring clans to the Umuike market.
In fact, the magic is so powerful that it even draws thieves to the market. For his part, Obierika maintains that only magic can lead to miracles; he certainly believes that it is the only reason the Umuike market has done so well.