If you cancel within 30 minutes of placing an order, it's canceled and refunded immediately, with no seller review needed.
After that window, you can still request a cancellation for any item that hasn't shipped yet — go to Your Orders and select Cancel items. This sends a cancellation request to the seller for their review, since they may have already started preparing your order. Once an item has shipped, it can no longer be canceled — you'll need to request a return once it arrives instead.