Game Play and Trading
One of the best ways to learn and practice the art of trading is via games. Not only trading simulations but general strategy and multiplayer games that allow trading of items. The reason I say this is more to do with the psychology of trading rather than the techniques used. When you play a game to obtain resources it represents work to replace those resources, so a losing trade means that you have to go and do more work to replace it. In other words there is some form of attachment to those resources. If on the other hand you run a simulation where there is no work to obtain the tradable thing then there is no perceived risk.
Initially, my favorite game for practicing trading was RuneScape, which is a large multi-player adventure game. More recently, however I have been trying out various other multiplayer games such as Stronghold Kingdoms and Team Fortress 2.