You should be able to contct the seller via one of their listings or from the dropdown menu arrow for the item on your purchases list. But if they don't reply or refuse to help there is nothing you can do about it on ebay. eBay only allow 30 days from delivery date for claims of any sort and they don't get involved with warranties.
If it is within 30 days you can open a return one ebay using the reason "Not as described". If it's more than 30 days and less than 120 days contact your payment provider and ask about a refund or chargeback.
