In the requests section you can announce your needs for certain materials, or fulfill the requests of other players. 

Suppose you absolutely want to expand some building, but you are missing concrete, and your own production is simply too slow to satisfy your own demands. Then you should announce your needs here. It's up to you to offer a price that is interesting enough for others to fulfill your demand! At least more interesting than the market rate and the price others will pay for it.

Since the price of goods can be driven to exorbitant heights with this system, it is always worthwhile to check this page, and send the goods to that man or woman who is willing to pay so much for them. 

Every player has a maximum of 10 active requests.

Only the best deals are shown for every item. If your offer is not good enough and you have too many competitors, it might not even be seen by other players.