My suggestion would be to slightly change the "Classifieds" section so that is is more like the eBay feel. By that I mean rather than have it as a 'forum' style where topics are created, have listings on the page that can be created, viewed and 'purchased' by members.
This would probably work best NOT to be done as an auction style however, instead fixed listings that members can create and of course once the item have been picked up and/or paid for, the listing can be removed (or ended).
It would make sense as it would also save people posting reply's to the thread for items that are old and no longer available, it would also make it a lot easier to use and clearer as to what is actually available.
If designed correctly, this would be relatively easy to implement and would result in very little maintenance from the mods. There would still be the issue of time wasters, but of course this issue is still present in the current system. However measures
could be put in place to cut down on this.
And of course the seller's rating information could be displayed on the ad's so that you can quickly see if feedback has been left from people selling things in the past. I know people offering services such as cruise control installments or retailing VagCom stuff would love the idea of this.
Anyway, its just a suggestion but I feel it is a very plausible one at that. Maybe someone up there will read this and agree?! If so then I would hope they PM me if they would like more information about how I could help