Once the community has many participants, you could consider paid memberships. You will likely need to offer something to make it worthwhile. This could be a private forum with exclusive offers and discounts or a marketplace for members to sell products (especially relevant for collectibles). I know of one forum where paid members can link to their website and ask peers to review. It is not for the faint-hearted, however, as the reviews can be brutal but (usually) helpful!
Advertising revenue needs plenty of visitors. Membership paywalls will restrict visitors. Thus the problem is twofold. How do you make a forum busy enough to keep people returning or worth paying a membership?
Spam is the one thing to avoid on any forum. That can be difficult, as once it becomes popular a forum will attract spam posts, such as thinly-disguised adverts and offers for prescriptions, passports, or bitcoin deals. A good forum is moderated so such posts are gone within a few hours at most.
Relevant to your products
A casual visitor will read interesting content. He may even ask questions if it appears the answers would be helpful. Again, the initial answers may have to come from staff, but over time visitors will ideally provide both the questions and the replies.
Launching a successful forum takes much promotion and nurturing. It will never gain traction otherwise. That’s why an active forum is rare. But, done correctly, a forum can generate customers to an ecommerce site as well as advertising and membership revenue.
What makes a good forum? In my experience, it is where participants can ask questions and receive relevant answers within a few hours, no longer. A good forum is busy with lots of helpful content. A high-traffic forum can generate a secondary income from advertising or membership fees.
Done properly, a forum can eventually be self-sustaining — requiring little staff time. It only works, however, if the forum directly relates to what you sell and interests visitors.
No spam
An ecommerce website can have content other than products for sale. Consider a forum, for example.
Loyal customers can serve as effective moderators. In time they “own” the forum and visit it more often if they think they are valued. Pitched correctly, the moderators can receive free memberships and nothing else. Moreover, volunteer moderators will not only remove spam but could also post knowledgeable replies.
Firstly the forum should be relevant to your ecommerce site, focused on topics that interest visitors. It should answer questions and provide advice. Ideally the questions, answers, and advice all come from visitors. But initially you’ll likely have to seed the forum with content and in-house members.
Achieving forum success is not easy. It will likely need considerable investment in time. But the rewards of frequent, interested visitors who value what you sell cannot be understated. You could ask their opinions, solicit pre-orders, and otherwise promote your products. Members could post about their products, forthcoming releases, business challenges, and more. Thus an active forum can generate relevant content.
