I regularly look(ed) at the old for sale forums. In a way they reflected the feel of the site. You were selling stuff, amongst like minded individuals, discussing it, having a laugh occasionally. I can't be bothered with the classifieds, it just doesn't work for me, I find it a bit clunky.
As for the fee idea. I rarely buy or sell on here, so it would depend where the fee was pitched as to whether I would advertise stuff I'd want to sell. It would certainly put me off putting things like, maybe, strings or pickups in the classifieds section.