I don't sell my posts, but i know people who do that. Also, i've heard about postloop, which is a service that pays people to post on other forums, but as far as i know, the price is really cheap, so i don't know if it pays off in the end.
I got here through Postloop. It's not good for making money, but I use it to generate unique content for my forums. You get credit for posting on other forums (like this post) which can then be used to buy posts from Postloop subscribers. Getting initial content for new forums is very difficult - most people will only show interest once there's a lot of content and daily activity. Postloop helps with that.