On a forum that I moderate, we allow advertising in the threads.
here is how it works.
lets take this advertisement for instance that involves a product to take care of rust.
the advertiser would send in the request and product info, then they would go down to the same area that we all go to to pay for our Premo.
there is another catagory there that collects a much higher, premium and gives them the title of sponser instead of premo. If they feel the need to have a private forum as an outlet to assist people directly, they still have to pay just like anyone else for that additional service.
for this they can step in and peddle their wares when appropriate, keep people updated on new product lines, and get to occasional banner add thrown in between posts.
so, even if they dont log in for a few days, if a thread is started that were to contain people talking about the rust on their horse trailers, maybe between posts 5 and 7 would be a bot generated post directing people to their business.
other than those benifits, most of them post just like everyone else, there is actually very little in the way of sales posts started and most of the people that post on the site will direct the new people to them when the need arises.
Of course this is a specialty forum that Im on so it may be different.
but in this instance, the poster trying to introduce their product could be limited to only posting in the horse areas, although, I see a greater use for a product like this besides horse trailers, you get the point.
Could end up generating a good deal of income for the person collecting the money from the memberships.