As a sidenote: Traders can contact the Banker directly of the Realm they are currently in.
yes. but they need to know there's stuff to buy in the 1st place before heading there or it's a bit of a wasted trip. by the time you get to write to bankers directly (as opposed to using old contacts or getting contacts from your banker) it tends to be a bit late.
bit silly to be able to write to a foreign ruler who's got nothing to do with trade but not the banker.
