Mel, this is a great question.
There is the Barter forum, as I'm sure you know. It's great in its own right.
But I sense you're calling for something a good deal more elaborate. There may be ramifications to setting up what would amount to a commercial aspect of such an undertaking, and I'm completely unaware of what those might be. I also suspect that someone would have to take on the responsibility to monitor and regulate a forum of that nature, and that would be a significantly large duty if it were to take off.
That said, the wherewithal to create a forum of that nature is out there. Two of the websites I use for tractor and implement purchases are instructive, one is commercial, one is not.
Moderators-- if it's inappropriate for me to post these links, please forgive me and delete them; I'm not plugging them, just using them for the purposes of demonstration.
The first site does pretty well for categorization, I think. It's easy for me to find my way quickly to what I'm looking for, and it's coupled with user forums not unlike what you see on HT. Here is that link:
http://www.tractorshed.com/cgi-bin/p...lassifieds.cgi
The other, commercial, site does a great job defining locality, at least in my opinion:
http://www.ironsearch.com/IRONSearch/content.aspx
I think those two approaches could be successfully merged. There would need to be some discussion of how to categorize things, what should or should not be offered, how one contacts a buyer or seller, how to deal with the inevitable shady characters, and so on.
But HT users are generally intelligent consumers, and many offer some very attractive items already, often by links in their signatures, so I guess what you suggest is already going on, if in a less-than-organized fashion.
Just my thoughts. Thanks for the idea.
Don