Basically, click new topic and add your headline of what it is that you are selling.
If you are creating a wanted ad, I would start off the topic as: “Wanted: 1957 Chevy fenders…”
Posting a for sale ad: Start it as “For sale: Used spray gun etc etc…”
Hope this helps. I think if we post clearly like this it will help keep it all organized. Also if you end up selling something, it may be wise to close the topic.