However, I don't really understand why something like Flathub is needed in order to sell it. That's not been a major issue in my experience.
Is the main point to provide a payment processing service?
Also this seems to be a confusion about what FOSS is. FOSS "freedom" isn't about "free as in beer", it's about "free as in speech." The GPL even welcomes commercial software, provided they include the source. I'd be thrilled to see a category of "paid FOSS" in the store, and I'd buy quite a bit of it if prices are reasonable and the software is useful.
I’d be happy to pay for Linux software but there is no way I’d want to be tied to a particular distro/repo/storefront.
There's basically Steam or DIY.
If they used the royalties to hire people to audit the packages I might even consider paying for free software, but I'm afraid it will turn into walled gardens where small developers have no say and the big ones just deal with the problems because it's the only option.
I hope it turns out to be better with KDE and Gnome that come from a FOSS background but money makes people greedy.