We knew that, and Elm knew that, but there’s no reason to assume the shopkeeper knew that. Unless there is, but I’m not gonna go rewatch the scene to find out!

My baseline assumption is that Ruche is figuring things out a bit later than Elm, so if she arrives at a given solution, particularly an obvious solution, Elm probably thought of it and then rejected that solution for some reason. Even supposing that this is not the case, Elm still rejects this solution and argues completely valid reasons for rejecting her solution.

As far as the shopkeeper is concerned, Elm is just another party leader trying to drive a hard bargain to maximize the party’s gold and her suspicion about Elm is that he is taking advantage of Ruche. The crone mentions this early on in their conversation and doesn’t believe Elm when he says it’s not like that.

When Ruche makes her suggestion and Elm refuses despite having wanted that skillbook, she revises her opinion of Elm and Ruche because it is then obvious that Elm is not bargaining in a way that is self-serving or even optimal. Her internal monologue stops short of saying this despite making basic observations and obvious deductions, but withholding the less obvious deductions is a something this show has been doing throughout its run.

I suppose it’s possible that someone might have to rewatch the episode. Or perhaps just rewatching the scene with these three characters might be necessary. Personally I find it funny that the audience is forced to imagine the mental state of these characters right at the point where we no longer have the benefit of hearing their internal monologues.