I have never installed the eBay app, so don't know whether anything has changed with that. Two possibilities occur to me:
1) There is something in the app's settings that means it is only checking when you open the app. As I'm not familiar with the app I can't make any suggestions for what that might be, just that you should check the app's settings for anything that might have this effect.
2) The app is being killed by some power-saving feature. Maybe try going into the phone's power settings and disabling any "optimisation" for this app? If you have a task killer installed (a third party app that kills apps in your background, also promoted as "RAM boosters", "performance boosters" or "battery boosters") uninstall it - those things are pure snake oil, and just conflict with the way Android works and waste battery power (but will waste even more power telling you how useful they are). If you have such a thing it could also be part of this problem, but get rid of it regardless.