Runnersworld's sister site 'BikeMagic' and also 'SIngleTrackWorld' (both mountain bike forums where I spend too much of my work day) both have both advertisers and 'classified' sections.
It makes more sense for bike sites (compared to a running site) though as there is more kit to sell on. I'd happily buy and sell frames, forks etc but not second hand running shoes and shorts etc.
I know this is posted in the 'TW Forum' and you lot do have bikes (albeit strange, uncomfortable ones) but it's still basically Runners World.