Was the product damaged in shipping, or faulty anyway?
You will need to talk to the seller and describe the problem, they may offer a refund.
If they don't respond, and you paid with Paypal, open a not as described claim (you have 45 days from payment to do this).
If it's been longer, talk to your bank about a chargeback for a damaged or faulty item.
You will have to send it back tracked if you try for a refund via Paypal (unless it's very cost prohibitive)/bank, so talk to the seller first.
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Buttercup: You mock my pain! Man in Black: Life is pain, Highness. Anyone who says differently is selling something.