a seller offers a five year guarantee with a purchased item

If a seller offers a five year guarantee it's entirely up to the buyer whether they can enforce it. eBay has no further interest once its own, 30 day money back guarantee has expired.

It's safer never to rely on warranties supposedly offered on items sold on eBay. Sellers' own warranties are only as good as the seller who gives them; we receive endless questions about sellers who don't want to know about problems. Even manufacturers' warranties are liable to be invalid for items sold here. Few sellers here are authorised distributors and some source their stock from grey imports or trade clearance stock of customer returns or seconds.

Remember that eBay warns buyers in the user agreement that they do not guarantee that items sold here will be truthfully described, safe or even legal. So what value would you place on sellers' claimed warranties?