You can trade a rare for 2 common resources, or 2 common resources for 1 rare.
The trading of 2 common resources for 1 rare is not specifically mentioned (that I can find) in the rules, but the trading post tiles do seem to indicate it is possible due to the use of the plus signs. Regardless, without your comment here, I would have assumed it impossible due to the lack of a rule mentioning it.
Anyway,my wife and I just got the game, but we love it! Thanks, Scott!