I turned off the Optimize Battery Usage for the ebay app and now the
notifications come up when they're supposed to. Otherwise it would only
come up when I opened the app hours after the item ended.
I think mine is the same (S9+ with new Android Pie). I opened the app
and got a notification that item ends in 14 minutes. Clicked
notification and said that the item ended over an hour ago! Happened
twice already and missed both items.
