Read Ebay's Money Back Guarantee (links on every listing, in various other places across the site, and on your order confirmation), and follow the clear instructions.
You also have the More Actions dropdown beside the transaction in your Purchase History, the Resolution Centre (link at foot of any page), and the Help pages with their integral search to refer to (links at top and foot of any page).
If you mean where does the refund go once actioned, it goes back to your original payment source.
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