The Marketplace is unlocked at City Hall level 12. This structure allows you to buy and sell grain.
How it worksEdit
You can trade grain by the hundreds up to an amount of grain equal to your level * 200 + 1000. Meaning a level 15 player will have a trade limit of 4,000 grain per day.
The trade amount can be reset by upgrading the Marketplace.
Grains price fluctuates, every 30 minutes it raises and lowers. Like gear upgrade success % (Magic), it will rise consistently until it reaches 2 silver per grain, and then lower consistently until it reaches 0.5 silver per grain.
The price change ranges between 0.05 and 0.28, meaning if the price is on the rise and currently 1.05, it can change to as little as 1.10 or as much as 1.33.
There's still more!Edit
There is a black market side to grain trade. The black market allows you to buy an unlimited amount of grain at double the current value.
A common strategy is to buy a lot of grain from the black market when the price is below 0.6. This puts the price at 1.0-1.2 silver per grain. When the price reaches 1.9-2.0 a few hours later, you then sell it, upgrade the marketplace, and sell more. Repeat this process as desired/able. You may overall profit less by buying from the black market, but the limit on grain trade makes it very worthwhile.