Assuming you agree that paying for some add-ons is ok then you have to ask what we do about people using AMO as a marketing platform. This is a tough question since we risk devaluing AMO as a website if it just gets filled up with adverts. I don’t believe that there is an official policy on this. It is such a rare issue right now that maybe one isn’t necessary, but here are are my thoughts on what such a policy might say.
In order to even advertise a pay for add-on the developer must have uploaded a real add-on. If this add-on is just junk, i.e. doesn’t really do anything and is just a means for getting a page onto AMO, then it should be removed. I have seen add-ons like that (that weren’t advertising anything) getting removed as a matter of course anyway so I think this is already working.
The next question is whether the add-on is a basic version of the premium add-on. If so then I think it is totally fair that the developer can include text in the page to note that a pay for version is available elsewhere. I’m less enamoured with the idea of including screenshots of the pay-for version on AMO. I think regardless of how well labelled they are there is a strong risk of confusing the user there.
Trial versions of add-ons pose a question. Should AMO be allowing add-ons on the site that will intentionally stop working after a period of time, or that list features in menus that only pop up a message about needing to pay to activate? I don’t think so. I believe a user should be safe to download add-ons from AMO that are whole products and they can count on to continue to work.
If an add-on on AMO is unrelated to a premium version by the same author then more care has to be taken with advertising different add-ons on the same page. I don’t see a problem with a short note at the bottom mentioning that other add-ons from the author are also available elsewhere, but I don’t think mentioning what the other add-ons are or what they do is a good idea.
Overall I think short notes are the way forward, anything where the add-on is more of an advert than an add-on in its own right is something that needs to be carefully looked at.
Update: A couple of commenters have raised the question of whether AMO should just include pay for add-ons in its listing, something that I completely failed to consider. I’m not sure which side I am on on that yet. I think if handled correctly it could work, but maybe I’ll think on it and do a follow up post on the subject.