What version of Android are you using? And what version of the eBay app?
See here:
https://community.ebay.com/t5/Android-App/Android-app-not-supported-on-older-systems/m-p/30652449#M9...
Sometimes uninstalling the eBay app, restarting your device, and then installing the latest version from the Google Play Store can help. If you are having problems getting the latest version, make sure you have the latest versions of Google Play Services and Google Play Store -- there is a link to clear your cache and update those at the above link.
If you are unable to update your operating system or the eBay app, you should still be able to use a browser to go to ebay.co.uk or ebay.com.
