I would like to propose that we add some sort of icon or badge to be displayed in the plugin listing, that indicates it is not free. Or that it represents a paid service.
Right now the overwhelming majority of plugins on the plugins database are utterly free. But there are others that represent paid services or require a subscription or license of some sort. My own Poodll plugins fall into the non free category. It would be up to the plugin developer/publisher to decide if their plugin is "commercial" or not.
The justification for this badge as I see it is:
i) it alerts the user that the plugin they are considering installing is not free or has paid components.
ii) it encourages the distribution of commercial plugins on the Moodle.org plugins database.
iii) it provides a possible revenue source for Moodle
To expand a little, its been mentioned previously that Moodle prefers to have a single plugins database (ie an official one and no unofficial ones). It makes sense then to cater to plugin vendors, to avoid alternate plugin marketplaces arising. My own experience has been that organisations want their key 3rd party plugins to be professionally supported and maintained. And I think a healthy commercial plugin ecosystem will be good for Moodle and for its users, and should be encouraged.
I also think that those making money from Moodle, or using Moodle.org as a distribution network for their product, should be contributing back to Moodle. So I think its reasonable to charge for the use of the "commercial" badge, or to require users of such to be Moodle partners, and possibly to specify guidelines for which a plugin might be deemed "commercial."
(Just because its relevant, let me say that I think that the collection of money from users is not a role that should be carried out on Moodle.org. That could skew things and would probably cause more overheads and hassles than revenue would compensate.)