While it's nice to be able to list items for sale to other users, it can be difficult when you don't know what anybody might actually like to buy.
For example, I would probably buy butter right now, because I want to be able to demolish and replace my butter factory, but not until I have a lot stockpiled. Sure, I can come on here, but then ten people might list butter and I really don't need/want THAT much.
So I'd like to be able to put up requests for particular items, perhaps putting the gold up front for the items. I think it'd make the marketplace a lot more functional than it currently is.