The buyer only needs to show they sent it back to you within 10 days, not that you received it within that time. The buyer will have to provide tracking numbers to paypal, paypal will then refund their money (which would have been on hold from your account as soon as they opened the case) and then if there is an issue such as the return never arrives or it comes back not in the condition sent, you contact paypal and you can, in some cases, claim back from them.