[Fantomen]

Remember that there is a value in the goodwill factor too. Selling an item for the base cost can be a good idea if you value the diplomatic relationship.

Other than that, I usually look for deals where I can trade something I need for something the other player needs. That way you can stick to the basic gem cost and still feel like both players had a benefit. (like I give you 3 air boosters for a ring of wizardry or something like that)