Is it safe to market on forums? My answer would be it depends how you go about it.
The first thing to do is to find some forums that have a direct link to any product you are promoting. Next read the forum rules. Some allow url's to be placed in your signature box, but that is not always applicable.
Always read some of the forum comments before you start putting anything on it. Perhaps there are some questions on the forum that you can answer. This would be a good time to put a comment. Your comment is going to be able to be viewed by all members of the forum so make sure of your facts before you put anything.
There may be a discussion going on that you can add relevant information to.