I think we're going to try without subcategories for now and see how much traffic the classifieds actually get and what impact they have on the overall feel of the forums. Since they will be more integrated into the forums, they probably shouldn't have a radically different structure from the other discussion forums, which aren't heavily subdivided. If they get enough posts to merit subdividing, we can always do it later.

Since they are now part of the forums, they should be searchable using the regular forum search tools.