Why not just add Iconomy?
It basically means you put the item in a box, put a sign above it (The same way you buy items from signs in the market)
but you actually have to have the items in the box, you put a X stacks per X money for X item 
It's not that hard, I'd be willing to build a massive trade district, and a person would have to buy a shop from the mods to sell their items in...
mod intervention is undesirable, inelegant, and not fun for the users or the mods.
we do not want to be reliant on third party plugin developers to create/maintain bug-free and headache-free code.
a trade district where people place their own chests / items / whatever would be a guaranteed eye sore.
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it's very easy to say things are "not that hard" when you're the one peering through the glass from the outside.
