Has anyone ever challenged that in court, if ebay allow sellers to advertise warranties then ebay are responsible to make sure the seller meets his obligation to said warranty.

Check section 2 of the legal agreement. eBay is simply a sales portal. It is not a party to sales made here and the seller is entirely responsible for any descriptions and claims they make. eBay even warns buyers that they do not guarantee the quality of items, or that these will be truthfully described, safe or even legal.

What shop could get away with stating this - and who would use it if they did? Many users have no idea what conditions they agreed to accept here. eBay is a jungle.

eBay's own involvement is limited to its own money back guarantee. This  only lasts for 30 days, and only covers the condition of the on delivery. When this has expired eBay has no interest, whatever happens.