1. Contact the seller, explain the problem and tell them that you want to return them for a full refund with return postage paid.
2. If they refuse or don't respond within a week then message them again, repeating what you said before and saying if they do not agree within a certain timeframe (at least 2-3 days, tell them the timeframe) you will report them to eBay.
3. If they haven't agreed within that timeframe then open a case with eBay. You should only do this after doing the above as eBay expect you to attempt to resolve the problem before opening a case.
4. After 8 days if they are still refusing or haven't responded then you can escalate the case and eBay will contact PayPal to reverse the transaction so you get the money back.
(The above assumes you paid with PayPal)