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Each shop of every type has categories that they can sell and they can sell only one category:: cheap items, normal, better, best and superior.
Unless the items sold are sold through the dialogue. Then you can mix types up.
By the way DemonLorddDiablo, regarding with finding the Spellbook of Calm, I've only just realized that there is one simple method of obtaining that Spellbook easily which I somehow forgotten about it.. 😅😓
In the "Mithril Horde I" expansion, there is a Spellbook of Calm that can be found at the Irrigated Fields. Just follow the blue arrow on the provided screenshot below.