The top icon shows how much merchandise is left for the blue player you can trade for. Every ship takes 20 away. This is shared between all docks belonging to a player and recovers slowly. Each player has their own merchandise counter, so even if blue is out of trade, you can still trade with red, yellow and so on.
The gold icon shows what is the exchange rate for gold per trip. In this case 7 gold for 20 traded resource. This will increase with distance and is based on your closest dock. So if you want the most gold you should trade with the furthest dock the player has and if possible delete your close docks and only leave the furthest one to trade with. This will allow you to gain the most return for your resources and their merchandise.
TL:DR Always trade with the furthest dock for each player and if more than one ship is waiting at the dock, you have too many ships trading with that particular player.