This is probably nothing to do with your app and everything to do with ebay, nonetheless i'll ask the question.
I have posted a couple of ebay ads on my forum using the external link option
https://arbtalk.co.uk/classifieds/item/1118-carlton-xl-sp4012-hydraulic-stump-grinder-2011-350hrs-no-vat/
The external link works fine on desktop, however when you click on it from your phone it doesn't take you to the ad. Try it yourself to see what I mean. Any idea why this happens or if there is any sort of workaround?