In this game app, players just buy and are unable to sell. Tradings are done at the Trading posts. Lower levels trading posts will have lesser number of available trades.
Here is an example of available trades at a trading post. Positive percentage shows that we will receive positive trades, i.e will get more resources. Resource icon on the left is the resource we take from our castle to trade with the resource on the right. So if you see negative percentage, you will get less lesser goods for a certain number of goods that you can take to trade. This amount will depend on the level of your.