Make sure you have a market built within your city walls. For each level of the market, equals one transaction at one time that you can do (i.e., level 2, means you can purchase 1 thing and sell 1 thing at same time). When the market is complete, click it. You will see tabs that say the name of the resource (i.e. wood, stone, etc). Click which one you want to sell. Within that tab, you see more tabs (i.e. Buy or Sell). Click the Sell tab. Put in the amount you want to sell, and then the amount you want to sell it at (i.e., 100,000 wood at a price of 1 gold per wood piece). So I am selling 100,000 wood for $100,000 gold. When someone agrees to that price, they will purchase it, freeing up your transaction spot.