You're wrong because all things sold from the market would go to the town treasury, not the player putting their things up, the items can be bought by anyone, residents of the town or not.
Yes it does mean people can put items to the minimum, but that means the town gains very little, people can already abuse town productivity by having alternative accounts so that isn't an excuse not to have this.
Example
Market: Players of the town may put items up for selling to anyone interested, all profits from the item go directly to the town treasury.
A minimum of the item would be how much you'd get normally for selling it to the normal shops, however the maximum price is as much as you assume someone will pay for the item.
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The only way to abuse this is by selling items to your friends for cheap prices and them buying them means your town gains some extra income to aid in construction of the buildings.
That is hardly abuse, if you say "but then people will make alternative accounts to XXX" well err...people can already do this as the only gain of this is the town treasury increasing, which I think the market would be a bit of a waste to create an alternative character over.
Edit: Oh and people COME to the market to BUY Things, that the residents have put up for sale, caravans for the market are pointless as you're not trading between towns or anything like that so quit trying to make it more 'complicated'.
, it'd be better if the trade was via caravan and the additional building "Trader" should be implemented as the town has space next to the mortician! Perhaps 10 levels and each level you get 1 more trade cart