If you have domains for sale then you can post them here for free. Just register and add one or more domains then add a description for each individual domain including price.
Benefit of joining is many including Discuss with others, chat via real life chat room either public or private, exchange ideas, promote your domain and website and many more.
Why don't you add unique forum articles with links to one or more of your websites. Unique because search engines don't like duplication. Be unique and share information.
SEO wise, you can advertise your website for free, join and start posting without being a spammer. Too many links from a page is spam. certain websites cannot be promoted.