After you have selected the item(s) you'd like to purchase, please proceed to the checkout page and enter the coupon code in the "Coupon, gift cards, or certificate" redemption field. Once you've entered the coupon code and click "apply" your total will reflect the applied discount.
If you are using a mobile device, select the "Gift Cards, eBay Bucks, and Coupon" on the "Review Order" screen. If a coupon is available for you, it will appear on this screen and you'll need to check-mark the coupon during checkout.