- Edited
I don't mean to be discouraging, but one of the worst things someone selling devices with GrapheneOS can do is sell near EOL or EOL devices to people. Nobody should be selling something older than a Pixel 6 right now, and that's cutting it close (https://grapheneos.org/faq#recommended-devices should be one's guide on which devices would make sense for this).
The reason for that is twofold. You're selling a phone to someone with the promise of it being secure when it cannot be past EOL. When GrapheneOS stops providing extended support releases for it (which are not complete, cannot be complete and do not make the device secure), those people are left stranded with an insecure device and without the technical knowhow to know what to do about it and what it means.
The second reason is admittedly more "selfish", but those same people may reach out to us, frustrated, because the phone they bought is no longer receiving updates, and why is that?
It's completely fine and allowed to sell devices with GrapheneOS provided you clearly explain that you're a third party and not affiliated with GrapheneOS, and that you do not modify the OS (as if the OS is modified, it is no longer GrapheneOS, and shouldn't be called that).