Every trade in the trader is assigned a star rating depending on how equal it is. That the item asked for is worth the item offered.
A 0 star trade is a very unequal trade in favor of the person posting the trade. 100 steel for 200 marble.
A 1 star trade is a slightly unequal trade in favor of the person posting the trade. 100 steel for 150 marble.
A 2 star trade is considered equal. 100 steel for 100 marble.
A 3 star trade is an unequal trade in favor of the person accepting the trade. 200 steel for 100 marble.
Unfortunately, the trader adds a tax of 50% of the item asked for when trading with other cities you have not yet discovered on the map. So, most of the trades you see in the trader will appear to be one star trades. Because, even though the person who posted it, posted it as a 2 star trade, the trader added 50% to the item asked for. Lets say you are near two cities A and B. You've discovered A, but not B. They both post the same trade, 100 steel for 100 marble. But to you they will appear different in the trader. city A's trade will appear as 100 steel for 100 marble. But city B's trade will appear as 100 steel for 150 marble. As you discover more cities you will find fewer 0 and 1 star trades in the trader. Unfortunately, as new players begin trading they see all the one star trades and assume they are fair. So, when the new player posts a 1 star trade of 100 steel for 150 marble, the trader adds 50% and it becomes 100 steel for 225 marble, which is a remarkably unfair 0 star trade. The best thing to do is post equal trades, and people will be more willing to take them.
When making cross tier trades it is especially important to try to remain fair.
Each type of good represents a tier.
tier 1: marble, steel, planks
tier 2: crystal, scrolls, silk
tier 3: elixir, magic dust, gems
The goods of tier 3 cost four times as much to produce as the goods for tier 2, and 16 times as much as tier 1. Goods of tier 2 cost four times as much to produce as tier 1. Therefore, the trader automatically adjusts the trading ratio when you post a cross tier trade.
100 crystals for 400 planks; 1600 steel for 100 elixir. (you can over-ride this by changing the amounts)
However, there is some debate among players if this represents a fair trade because, while it does take into account the additional cost of production, it does not recognize the time required for production. It may cost the same to produce 1600 steel as it does to produce 100 elixir, but it, generally, takes much longer to produce that much steel vs a relatively small amount of elixir. For this reason, when players post cross tier trades they should be 3 star trades to acknowledge the production time discrepancy.