Here is my little idea regarding economy.
Since trading big amounts of stuff might be time consuming, as well as storing non-building materials in larger amounts, I would like to propose a "stock exchange" building.
The basic idea of it is that you can store any amount of any material in it and it gives you back a parchment note which would grant you the right to take those materials back. Not only in that exact exchange, but in others as well, provided they have those materials in storage.
Such structure should be usable even to people who do not own the land it is on in any way and should not be attackable or being stolen from in any way. Maybe to do that, you would need a special skill?
I do not know how to deal with quality. Maybe an issued note would grant a right to get items of a certain quality range? But that would make speculation possible...
Also, I think this might be a good addition to the economy, since notes could function as paper money.