Scroll down to the Swap Meet forums, pick the appropriate sub-forum for the item(s) your selling, make a new thread. Scan over the sticky threads at the top of each forum, that is where the rules/guidlines are laid out.
Describe your junk (pics are a plus), tell where its located, list contact inforamation, pickup only/delivery available, etc. Be sure you post a price, without that, your thread will get yanked.
As a non-supporting member, you are only allowed 3 posts per sub-forum. If you'll be selling more than that, you might as well get a full-blown membership, click the SUBSCRIPTIONS tab all the way up top for details.
Good luck with the sale!