What they really need instead of subforums is catagory filtering. They decide on keywords for each forum (so in this case "For Sale" and "Trade") and then allow the user to mark their posts with any or all keywords that apply. If you click on the FS/T forum you still get every post made in the forum, but if you click on the "Trade" keyword it filters out all posts except those marked as "Trade". If your posts falls under both, you mark it as both.
The only problem is that moderating that would be a pain in the ass.