I used to do that as well (on Google Groups & some Joomla forums). And I used Discourse on a few popular JavaScript forums. So, I have some experience of both.
In practice, people really value the effort we put when helping them. At least your potential buyers. So, they will find you from your profile. And when you are answering use Youtube videos you created or your blog posts.
That gives you ROI rather spamming every message with an ad.