Just tried it out with GrapheneOS on my Pixel 7 and the eBay app works fine.

Generally I think the marketing of GraphenOS is interesting. They are usually positioned as the "Absolute security. No compromises." ROM, but in my personal experience, they are the "It must work. No compromises. Then make the rest as safe as possible"-option, given aspects like using the "real" Google Play Services (if desired), but sandboxed, instead of MicroG or unrestricted Google Play, which pretty much all other ROMs roll with.

Well, eBay does not work on my Pixel 6a.

I don't have it with me, by Play store says that it is incompatible with my device.

Google searches indicated that this is a common problem.