For non commercial advertisements, the Swap Meet forum is the place to post. This forum is more or less like your neighborhood flea market - buy, sell and trade with the locals. You're allowed to post here if you're not making money from what you're doing.
For commercial advertisements the Vendors Bazaar is the right forum to tell the world about your products. If you make a commercial post elsewhere in the forum, it will end up being moved here. It's currently free to post in the Vendors Bazaar.
If you would like your own private forum dedicated to your business, you might like to join the dozens of vendors and manufacturers in our Commercial Sector who interact directly with their customers on diyAudio. Contact us for details.
We also have many other ways to promote your business including banner advertising, our bi-monthly newsletter and social media channels. Contact us for details.