You start with 4 Slots and can purchase 8 more for a total of 12 Slots. It costs 12,750 coins to purchase every Selling Slot. When an Item has sold the icon will change to an envelope with an exclamation point. To cancel a sale press and hold on the item.
At the bottom are four Slots for visiting Customers. There can be more than four at a time but only four will be displayed at a time.