The eBay app is listed among the apps that block GrapheneOS (see here). That doesn't seem to be the case (anymore). I successfully installed it yesterday and it runs without any problems. In fact, I had already tried this a few weeks ago, as expected, without success – perhaps the large marketplace has actually changed something here.
If anyone else can confirm this, it would make sense to remove eBay from the list.
Thank you for your fantastic work!
HTH
Gregor