In the Wildmart, you can find all the items from other people's Stalls that are for sale. Basically any item that anyone else is selling, you can find it here. It can be found in Wild City. It's a good place to come if you want to buy something quickly.
Browse through the categories then Enter an item to view all the sale listings for it, including the seller, their stock (amount of the item for sale) and price per unit. They are ordered from least to most expensive and you can scroll down. When you press Buy you can choose the amount you want to buy (if the seller has more than one for sale) and press OK to confirm your purchase!