No idea what kind of demand there might be for such a feature, but the ability for non licensed users to pay for add-ons would be very helpful. Even if it has to go through the licensed user. Maybe VIA a link that the site owner can share?
Most paid addons are hosted on 3rd party sites, and your registrations on that sites are independant from xenforo.com registration. Don't see what the problem is.
No, no problem. I wanted to be sure what you ask. If I understand you correctly, the feature you are looking for is already there.
Just a reminder. If you have a XF license (doesn't need to be an active one, can be an expired one), you can access your customers page and click on "forum users".
There you can associated forum user names with that license, example for me
That means you can also list other forum user names other than yours. Those who are non-licensed users. Then they would also have access to the Resource Manager and can use it like anybody else and be redirected accordingly when they click on that paid addon's button.
Correct me if I am wrong @Brogan, don't want to give wrong advice here.