What version of Android are you using?
See here:
https://community.ebay.com/t5/Mobile-App-Android/Android-app-not-supported-on-older-systems/m-p/3065...
If your Android version is recent enough but you are still having problems getting the latest version of the eBay app, check to make sure you have the latest version of Google Play Services and Google Play Store.
If you cannot get the app to work with your device, you should still be able to use a browser to sign in to ebay.com.au.
