There are different types of forums, beyond the obvious topical types, of “marketing” or “special interest”, etc. “Marketing” and “special interest” are two types of what are commonly referred to as “open forums”.
There are, however, business and professional forums, which do not operate under the same rules and, more often than not, disallow any and all types of advertising or self-promotion. Forums that have been created to support the membership of a particular program are especially adamant on this topic and often demand that all members refrain from all types if advertising on their sites.
Still, even with all these hurdles, it is possible to successfully advertise in forums. In all cases a pre-requisite for success is developing a good reputation and a good relationship and maintaining both. So, while it is true that this a form of free advertising, it does require a certain amount of investment as far as time and energy are concerned, not to mention subtly and finesse.
To successfully market on any forum, the first requirement is to take a personal interest in the main topic of the forum. That means visiting it on a regular basis and developing a good relationship with the members and the moderators. It, also, means taking an active roll in the conversations and being willing to help others. Of course, it goes without saying that it means abiding by any and all rules that exist. In this way, one can develop a reputation, and business will just naturally develop because humans tend to work with people they trust.
Since the main purpose of a forum is the exchange of information and/or ideas, marketers must respect that objective and abide by it. Marketing forums, where everyone there has something to advertise and sell, usually have even more stringent rules.