The items you buy in the game (clothes, jewelry, accessories, or hair color) are immediately added to your wardrobe, and you don't have to repurchase them. You can access them in the store by clicking the INVENTORY tab next to the SHOP options, and you can use them several times.
However, you might be asked to renew/glam up your items if you don't have enough new garments in your look to meet the Glamour Points requirements for a specific challenge. Every item renewal increases your wardrobe value.