Just like when you go shopping! Anyone that has earned enough coins can purchase these items. These are available to everyone on a first-come basis. Consider setting the Cost relative to the “real value” of the item and how long it takes the average user to earn enough coins to make a purchase. Click on the blue graph beside the Cost field to see how many coins are being earned per day/student.
Fill out the rest of the form as needed. Choose the redemption method to determine how the user will acquire the item. Click Save to post the item in your store. Here's an example of an item set for Purchase: