Yup. Latte has mana but it is extremely rare in that world. Arwin, magicians and the Prince are the only characters who are able to use magic (as far as we know) I am assuming that she has magic because she transmigrated to that world. Since most people can't use magic, the shopkeeper thinks that Latte is a gullible customer like almost all other customers who buy scrolls.
It could be, since she's otherworldy, that she has a lil' extra from beyond (the mana), but that wouldn't help the narrative in any way, it'd be maybe an overlooked detail. And the author really owns her narrative, from the detail to the overall flow of the story (well, I do think the Prince should make an appearance soon or he'll be forgotten).
I still think the word itself is used with a purpose that will be revealed further down the line. Because, as I said, the scrolls' value and utility check out, they do work and they are indeed expensive. But in way is she being fooled?
For a user of magic devoid of mana, a scroll is a perfectly adequate tool. So if you'd call this user names, you'd maybe say ''poor you'' or I don't know, ''mana-lacking money cow''. But not gullible; more so when the author is pretty savvy at her choice in words. But who knows?
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Ok, remember when Arwin was floating along, minding his own machiavelyness and saw Ibelin and Latte on that alleyway? He noticed Latte's mana. Mana is something that not every character has - the magicians painstakingly imbue scrolls with it, the knight also uses it to freeze enemies of all forms in their places. But that's about it. Also, at the scroll shop, why would Latte be (almost) called ''gullible''? The scrolls work, and it's clear to all they're expensive.
Could it be that she's gifted with some sort of power of her own that, once mastered, would render the scrolls useless?