The subject line for your ad MUST begin with the prefix "Free:" or "Trade:". When you create your ad you will see a Prefix menu to the left of your post title. Choose either Free or Trade, whichever is appropriate for your post, and the prefix will appear in front of your title. You do NOT need to put the words free or trade in the actual title, just choose the correct prefix.

Examples of how this will look:
Free: widgets and gadgets
Trade: widget A for a gadget C

This way users can quickly distinguish the free ads from the for-trade ads. Also, by selecting the appropriate prefix in the display options at the bottom of the page the users can quickly display only the Free stuff or stuff for Trade.