In the town of Prima, there is an empty counter inside the building that has three shops set up, one by one, next to each other. If you place Gobi behind the empty counter, and press the 'talk button', he will set up shop and customers will start flowing in. A customer will either offer to buy or sell an item from or to you. Occasionally, rare items will be offered to you, so make sure you have enough money to buy them! For a complete list on the items that can be offered to you at certain points in the game, take a look at Gobi's ability page.