Interaction is a good thing, but I'm experiencing the lack of answer very common in the game, that also one of the rights of the players.
As steward, I'm trying buy food in EC and I sent many letters, receiving no answers. So, I think that a system that allow you to message the lord will be nice, but only will get result with those who care in answer.
I don't have the same time playing this game as others here, mas in my time playing this game I saw that things not mechanically supported or militaristic, will fall in the mists.
So, for this to work, in my opinion, there must be an option of "Bargain" or something like that where the trader make an offer, as when you change your share, and a message appear to the seller, or buyer, informing that he have a deal to look at. After sending the offer, the trader would have the possibility to send a letter.
