It can't be done.
That said, here is how one could do it.
You (person A) have a character on Server A. Some other schmoe (person B) has a character on Server B. To trade items from Server A to B, you would have to create a character on Server B, while person B has to create a character on Server A. Then, you give person B's new character the stuff on Server A, and person B gives your newly created character money/items on server B.
Clear as mud?