It depends on where you post and how you post. The same with mailing lists.
Some lists specifically permit advertising under certain conditions.
First, a substantial number of the people who are interested in your service will, either accidentally or intentionally, direct their responses back to the group instead of emailing you anyway. Third, why should you be adverse to simply going in, participating and contributing. These are, after all, your customers. If you don't make an effort to get to know them, you're missing out on the biggest advantage of being on the net. Aside from the discomfort flaming might generate, there is a further danger that if half of the list is flaming you for posting inappropriately, the other half will be turned off of using your services.