How do I post in "Free Stuff" or "Swap / Trade"?

All posts with a price of Free appear in the "Free Stuff" category, regardless of the category they are posted in.  Similarly, all Ads with a price type of Swap / Trade appear in "Swap / Trade" category.

So to post in the "Free Stuff" category, click Post Ad in the header, select a relevant category and then select a price of Free.