You may cancel the sale from your swap page. Please ensure that you've communicated with your buyer before submitting the refund if you haven't already.
From the web version of SidelineSwap, merely click "Refund Purchase" at the bottom right of your swap page and submit the correct refund reason.
From the App, click the three dots at the top right of your swap page, select "Refund Purchase" and submit the correct refund reason.
Please note that negative feedback will be generated unless the buyer has submitted the cancellation request or if it's a bundle request. In the interest of avoiding disappointing buying experiences, it's very important that you keep your inventory up to date.