It's a tough one and I do sympathise. Selling designer items is a minefield, because there are so many fakes out there and some of them are so good it is hard to tell the difference (not that I am suggesting for a moment yours were fakes)  That's why  in these kind of disputes Ebay usually puts the onus  on the seller to prove authenticity e.g. a receipt from a accredited retailer.

When you say I have not received the shoes back despite claim for tracking.  Do you mean the buyer is claiming to have returned them? If this is the case then I would certainly be taking it up with Ebay.