eBay guarantee is most reliable as they will provided the replacement..and on bright side most of the time they will allow you to keep the product with full refund
You could always check your purchse number in the Purchises section or see the info about the profile. Above all in the menu is your login, and to contact the support just press the HELP&Contact - on the top right menu dots, if you are in the mobile app. They will help you to identify the account number.
Ask the ebay seller if it was brand new and if there's a warranty. If so how would they suggest handling the warranty. If it's a recent purchase they MIGHT exchange or refund the purchase price.