If your Android version is new enough (6 or higher last time I checked), you should be able to get the latest version of the eBay app (6.7.0.10) from the Google Play Store.
https://community.ebay.com/t5/Mobile-App-Android/Android-app-not-supported-on-older-systems/m-p/3065...
If you cannot seem to get the latest version there, make sure you have the latest version of Google Play Store and Google Play Services:
https://support.google.com/googleplay/answer/9037938?hl=en
