The IMF (international monetary fund) is responsible for the stability of the world's financial system - by providing stability, it encourages the trade - it assures the members that they won't loose their investments.
The World Trade Organisation has a more direct effect on the trade. It directly regulates (and sometimes monitors) the trade, making sure that the trade is free and the rules of the trade are predictable. The WTO also deals with any conflicts between the parties involved in the trade.
