Okay, I looked into your issue and found the following:
Only forums that are accessible for guest users can be read by an external RSS reader - unless the reader can take your login credentials.
The other forums that work, are publicly accessible. The FS/FT section is for members only and thus not available for an external reader.
If you enter the FS/FT forum and click on "Forum Tools" in the top right, then "Subscribe to this forum" it will give you options to get Daily or Weekly emails.
While I don't think this is what you were looking for (you probably want something that updates quickly for new listings), it is an option to get daily/weekly summaries.
You may want to look into an RSS feed reader that supports RSS authentication. Please keep in mind web-based readers will cache your credentials. Just something to keep in mind when handing out your login info to a third party.