EBay and Paypal's Buyer Protection program ends after 45 days.
You cannot post feedback after 60 days.
If either you or the seller has already posted feedback, you can add a Response indefinitely.
I've been here since 2001 and feedback has been 60 days in all that time. The indefinite Response is relatively new, maybe five years.
However.
Most eBay sellers are not agents for the manufacturer. And most warranties are from the manufacturer.
If the product failed, you should get in touch with the people who made it.
Be aware that most manufacturers will not stand behind their products if the product was not purchased from an authorized dealer.
It is also possible that the product is not what was advertised but a counterfeit. This is particularly true if it was made in China. The manufacturer in that case may be registered with eBay's VeRO program.
Five months is a long time, but you could also try calling the credit card you use to back your Paypal account. (PP is just the mediator) Ask about chargebacks. While most chargeback programs end after 90 days some are longer though rarely more than 120 days.