Right it seems people are not quite sure how to do this. So here is a tutorial. This is the quickest and most effective way of reporting a spammer or any other selling/offensive post.
1) Look below there username for this icon:
2) Click on it for the post in question. This takes you to the report page (see attachment).
3) Fill out the details as you would a normal post and click send report. This goes to all the moderators and one of them will deal with the post.