Not here to argue, in my mind I just had another idea. :3
Instead of whisper interface a merchant like feature, i.e. you /afk and your message or something, then others would be able to go up to you, right click you and a merchant window opens with that persons merchandise.
The trading wouldn't be possible, but he could add his orders on your log, for you to check later and go trough and service.
I actually coded this for fun but that was all it was, for fun.