Candy is used for a lot of different things in your pawn shop, from dealing with customers to purchasing better equipment. Most purchases in the game are made with cash, but the best of the best is always purchased with candy.
Bring back customers if they get fed up while haggling, or entice new customers to your shop if you run out.
Reduce sell times for items with no offers, or instantly get a new offer if you don't like the current one.
Purchase the best shelves, improvements, and decorations in the shop.
Hire the clerks with the highest stats.