This building can help with trade between future settlements or towns. To build this you need village dominium or town, People can leave items to sell for other items or money. Merger dominium to create a village or town for example 4 large dominium creates village and 8 creates a town.
Marketplace, money nad merger dominium
I do not mean marketplace what is in wow, not global, just in your town. Some one can come and see what is to sell in your marketplace or what u need to buy. Someone can buy items from you or sell you some when u are offline, it will be easier then looking someone chat. Imagine that big town lots of people u need iron and you write on chat, if you find someone you have to go to him. Now when marketplace is in town u can see what items is in to sell, remember to sell items inn marketplace u need to put items there(ore some other mechanics dont know yet ;D). Look someone want to be a trader and he go from one marketplace to another, buing and selling. Ijust think of the future when group off people build of town or village people need a little economy
I see this similar to Lineage 2 shops.
You are building "shop" where you can set like "Sell 10 iron ingots for 50 apples" So everyone can come to you, see what you are buying/selling. Then they can collect apples, or sell wood for apples, to be able to buy iron ingots.
I think we have a very similar idea, @Dybid.