Unless you don't want to deal with it at all. You can't "do it once", you have to keep doing it, over and over and over again. And if you stop doing it, then game forces a negative interaction between players: "Do this or else". If the selling lord was actually doing this intentionally, then fine. But most of the time it's a scenario where neither the seller or the buyer actually want to deal with it, and just want to move on and do things they actually *want* to do. This is what I meant by forcing a negative interaction. It's something that neither of the two wants to do, something that the game could easily do for them, and for which there is no good reason that it can't be automated.
I don't have a problem with that, either. If the lords want to gang together and purposely starve the city, that's a valid interaction.
I must have expressed myself badly. I fully agree with what you said.
