There is also nothing stopping people listing things in the classified but then having a For Sale thread which links out to your classified listings. Getting you the awareness of a For Sale thread but also having the ease of a classified listing facility.
I love scrolling through the classifieds - over the years I have purchased quite a few things that way
Maybe a temporary sticky at the top of the For Sale forum just reminding people of the Classifieds as well as linking to that section of the board may want more sellers to use them?