Settlers of Catan FAQ - Trades
Q: Can I give cards away? Or trade them in exchange for another player doing something I want (such as not moving the robber next to my city)?
A: A legal trade must include an exchange of Resource cards between two players. Each player must receive at least one card in the exchange, or it is not a trade. You can not give Resource cards away. You must get something for them. Similarly, you cannot give another player Resource cards as a bribe, because you would not receive any Resource cards in exchange.
Q: Is this a legal trade?:

Player A offers Player B (who owns a 2:1 sheep port) a resource of his choice if he agrees to exchange two sheep for a resource he needs.

A: The answer to your question depends entirely on whose turn it is. B player may only use a harbor on his own turn. So, in your example, if it is player A's turn, then player B can not use his port to trade the 2 sheep for the desired resource. However, if it was player B's turn, he could use the port to transform the 2 sheep into the desired commodity.
Q: Are players allowed to suggest trades when it is not their turn?
A: Yes. The rules state that you may only trade if the player who is taking his turn is included in the trade, but there is no rule that states that you can not negotiate a trade when it is not your turn.
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Copyright Wednesday, June 05, 2002 Mayfair Games, Inc.