Illidan will not be able to trade items with Malfurion, I consider that a bug and eliminated it. However, you will get a load code for Illidan and Naj'entus on Sentinel 8 that you can use on Alliance 5 to load Illidan's items and transfer Naj'entus' items to Akama. (Or into a Stash if you haven't convinced the Dreanei to join you just yet.)