There are a couple of exceptions. If you have a warehouse full of computers or bikes and you're in the business of selling them, that's probably over the line. A second exception is on the Web. If your company has a homepage, websurfers who call it up would be offended if you didn't have information on your products, distributors, and so on. Just make sure your posting is information, not ad copy.
Deception is another matter entirely.
There's a new form of advertising that's showing up on some of the musical newsgroups. And all the artists he raves about record for the same major label. It's not restricted to musical groups, either. Shortly afterward, person B replies that the new offering P from R Software solves that problem, is cheap and easy to install, and everybody should have one. A reappears with another question, and sure enough, product Q from R Software is the answer to the world's ills.