23-05-2020 7:27 PM - edited 28-05-2020 7:24 AM
Ebay can have no idea what you actually sent as they were not present when sent or received, so have no choice but to accept a buyer's word for it when they claim Not As Described, as per their Money Back Guarantee. They cannot get into disputes between you and a buyer. Only a judge can decide if a buyer is a liar or scammer.
You agreed to comply with the MBG when you registered your account. If you dig your heels in and refuse to offer refund on return, Ebay will either force you to pay for a returns label, or they will force the refund without the need for return. You will also have a damaging defect slapped on your account for failing to comply.
This may sound unfair, but it's the fairest it can possibly be. The MBG is a very good thing (although you may not think that at present) as it gives buyers the confidence to shop with you and every other Ebay seller. Without it, you'd be lucky to sell anything at all. In fact, without it, Ebay would no longer exist.
As you failed to agree to refund-on-return, you will have to deal with this outside of Ebay. Start by sending the buyer a tracked postage label (you can buy one from any carrier online, and email it to the buyer), and tell them they have x days in which to repackage the shoes and get them in the post, otherwise you will be forced to take legal action. If they fail to do so, send them a 'before action' letter by Signed For post, giving them a fixed timeframe in which to return, otherwise you will take them to Small Claims court to recover your loss and all court and other fees. If they still don't act, take them to Small Claims court. The process is simple, and can be started online.