I have the "Subeta Travel Agency" gallery and most items are/will be in their place, like Arctic Frost, ShadowGlen, etc. But I don't know where to place the NPC items from the main shops, like Kain sticker or Mauve TC? Are they in Centropolis?

As far as I'm aware the "main shops" don't have any location in the lore. You could create another category with "shops" or possibly create your own lore of a mall complex with no known location. Alternatively, you could put them in Sacred lands, as part of the Souk, with Cavern of Curios and Spells galore.

Awesome ideas, thanks! I think the mall "speaks" to me louder, since I love it in real life too :D