How does sending back a faulty item via paypal work? - Buyer protection?

Ok I am waiting to be refunded for an item I have bought on ebay, sadly the item is not fully functional and the seller refuses to refund me so I had to go ahead via the paypal route. What I don't understand is- If I send back the item recorded delivery but the seller denies and says for instance that the item... show more Ok I am waiting to be refunded for an item I have bought on ebay, sadly the item is not fully functional and the seller refuses to refund me so I had to go ahead via the paypal route.
What I don't understand is- If I send back the item recorded delivery but the seller denies and says for instance that the item I send is not the same or I have send a comepletly different item, how will paypal solve the matter?

I am not planning to trick the seller by sending something else in order to keep the item and his money- but how would paypal deal with the matter and how could it be proven?

thanks :D
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