At lower levels (prior to level 30) you start to get a false sense that you have a ton of extra crops and iron. In fact, prior to level 30 you'll have probably 10 times more iron and crops than timber and stones and it will be tempting to sell it off to try and get a little bit of extra money here and there to buy more timber or stones.
Don't sell that iron just yet, or your extra crops. Why you ask? Because the second you hit 30 you will regreat having ever done such a thing. It you sell it off now it will greatly hinder your advcement from level 30 to 40.
Long story short, selling off ANY resources is a huge mistake. Don't do it!
I promise you, you WILL need those items soon enough and in vast quantities.