You message the seller through eBay, all you need to give them is the email address you use to receive payments. Nothing else. However, they should have that already if you paid them. The seller can go to the items he sold, select your item, View paypal transaction, then it will redirect to his paypal account, at the bottom of the page it will say ISSUE REFUND. He can click on that and complete the refund. He doesn't neeed additional info from you.
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