This quest is available starting Chapter 19.
Go to the northern area of the market. There should be a stall nearby that you can actually go behind it. Talk to the store owner. He'll tell you that he is going to be busy and wants help for a short time. Accept his request, and you need go behind his stall in seek mode and target the customers walking back and forth.
Women will buy weapons, men will buy arrows/armory, and elderly men will buy materials.
Note: This sidequest will be unavailable after Chapter 31 (Lockout).