Honestly, the best place to sell your product is wherever people who are interested in your products hang out. For example, if you sell a product about how to cook mexican food, cooking websites (including forums), recipe sites, etc would be where you would want to go and advertise your product at.
You wouldn't go to a fishing forum (unless the site was about how to make fish enchiladas or something) and sell something related to your ebook on how to cook mexican food. Marketing isn't about spewing your product everywhere you can, it is about strategically giving people who are interested in your product the needed information to convince them to buy it.