I agree. On the surface, it seems like a cool idea (I actually brought it up a year or two ago), but I could see it degenerate into a big, out-of-control thing.
Ideally, everyone here would be cool/mature enough to post something (either looking to sell or buy) and only legit, interested parties would respond and they could work out the details.
But too many troublemakers, pot-stirrers and riff-raff here at AI, and they'd just mess it up.
We've all got each other's e-mail or PM info.
If a member is looking to buy or sell something, maybe he can post a quick note in General Discussion or somewhere, then the interested parties can communicate privately via e-mail or whatever.
Probably the safest, most low-key and hassle-free way to do it.