I mean, f-droid allows adding custom repositories that don't follow the main one's rules. There's a decent amount of apps that do this for one reason or another. Other apps, like for example OSMAnd, have a special f-droid build with google services removed, leading to the lack of some features like android auto support.
The client doesn't support any form of purchases and I don't think it ever will. I think you'd be best served using any of the other stores that support this or hosting it on your own website.
I don't see why fdroid would never provide a payment feature. At least for ethical reasons. Fdroid focuses on free software (and is also concerned with privacy and other features). Free software does not need to be gratis. As long as the payment code is open source. At worst it would be a NonFreeNet anti feature. Could be mitigated by supporting several payment platform.
You're right, but I don't think there's any incentive to do it. They pride themselves on having no tracking or otherwise user information (not even app popularity statistics), and adding any form of payment would require managing user accounts or coming up with impractical multi-device proof-of-purchase sharing solutions.
It's also worth mentioning that since everything on the store would still remain free software, making it legal to share purchased copies, people seeking to use it as a business model (like the parent post) would generally be ill-served.
The client doesn't support any form of purchases and I don't think it ever will. I think you'd be best served using any of the other stores that support this or hosting it on your own website.