For each resource bought, the player must give 2 coins to the owner of the resource.
Selling a resource to a neighboring city DOES NOT prevent a player from using it, during the same turn, for his own construction purposes.
It is possible, during the same turn, to buy one or more resources from both neighboring cities.
The resources bought can only be used on the turn in which they are bought.
Players can never refuse to sell resources.
Some commercial structures (yellow cards) reduce the monetary costs of buying resources from 2 to 1 coin.
If both cities neighboring a player produce a coveted resource, that player is free to purchase from either player.
To buy resources, the player must have the coins at the beginning of the turn. The coins earned through commerce during a turn cannot be used on that turn, only during the next one.