What version of the app are you using?
Is it the latest version for your operating system?
Try restarting your device between uninstalling and reinstalling the eBay app to ensure a clean install.
For more detailed app help, try posting on one of the app boards:
https://community.ebay.com/t5/Mobile-App-Android/bd-p/androidapp
https://community.ebay.com/t5/Mobile-App-iOS/bd-p/iosapp
If you cannot get the app to work, try using a browser to go to ebay.com instead.
