Trading is one of the most vital parts of Forge of Empires. Since a player can't find Deposits of every single one of the Goods, trading becomes a huge part of the game when you need goods that you don't necessarily have easy, cheap, and timeless access to.
Of course, a player can produce any of the Goods so long as the player has unlocked the necessary buildings to do so. Note however that unless you have access to a Deposit, the rate of producing the Goods will be much, much slower. For example, with a Deposit you'll recieve 5 of the goods over a time of 4 Hours. If you don't have access to a Deposit, you will only recieve 1 over a time of 4 Hours. Not very fast nor cheap, huh?
That's where Trading comes in, other players will get access to other Goods - and eventually you'll be in need of their goods and vice versa, they'll need some of yours. So one of you creates an Offer, as seen on the screenshot to the right, you'll have a list which shows what Goods you have and the amount. You simply open the dropdown boxes and select which resource you want to offer, and then what resource you want to recieve in return. And of course the amount.
Most offers are usually a Ratio of 1, meaning you give the amount you recieve, in example 10 for 10. People tend to have different ratios if the Tier of the resource is different, for example 5 Jewelry for 10 Lumber and so on and so forth. Of course this varies between continents.
I don't think there is a limit to the number of trades you can have at one time. I have had 4 pages worth before, so that should be sufficient.
You are not allowed to initiate a trade with a ratio of less than 0.5 or more than 2. Note that to get your offer accepted, you will probably have to beat the neighbourhood competition. This is because when opening up the market, the most profitable trades for the recipient are displayed first, and so that is what they will probably take.
Trading with a member of the continent who is not in your guild will require the recipient of the offer to spend a forge point on the transaction. Within guilds, trading is "free," only costing the outlined goods.