If that means you just bought something you no longer want, you need to ask the seller if they will cancel the transaction.
Contact them through ask a question on the listing.
They do not have to.
If you want to decline an offer or a counteroffer, you should be able to do that via the email you will have received about the item, or through the item's details in your eBay page.
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Buttercup: You mock my pain! Man in Black: Life is pain, Highness. Anyone who says differently is selling something.
