で vs に for locations
I know that for actions, で replaces に to indicate locations.
レストランで 食べます。(I eat in a restaurant.)
Still, I am puzzled by this:
そのみせで いちばん高いおかしが ほしいです。(I want the most expensive snack in the store.)
So is "wanting" an action, then?
I want to thank Faisane specially for raising questions from time to time that help me think a lot.
Concerning this question in particular, if we reduce the sentence to its radical terms we have
The words before お菓子 are added to enrich the description of the cake. So it is not necessary to consider whether it is で or に within this part for anything after お菓子. (Side note: 欲しい is an adjective but not a verb, and です is only added to enhance the politeness and is otherwise not necessary.)
We want to describe the cake as "the most expensive" "in that shop". "In that shop" defines the scope of "the most expensive". When we set a scope for an adjective, で is the preferred particle.
｛【そのみせ】で 【いちばん高い】｝ 【お菓子】
日本で いちばん高い 建物 は どれ ですか
お家族で いちばん高い 人 は あなた ですか
It is one of the several usages of で. Not all usages are a matter of the type of verb used. Check out this short article which I think concisely and effectively explain them in a nutshell
The use of scope as an adjective (which I still don't know enough about for the moment) and the given examples seem closer to Faisane's sentence than the extended version that I speculated. Your note was very interesting.
Just one small note: I think the sentence お家族で いちばん高い 人 は あなた ですか is not quite correct with just 高い, and probably should be something like お家族で いちばん背が高い 人 は あなた ですか or (most appropriately) お家族で いちばん背が高いのは あなた ですか。
My initial guess when I read the sentence was that it conveys a shorter from of:
そのみせで 売っているおかしの中でいち高いおかしが ほしいです
which may be translated to "I want the most expensive snack among those for sale/being sold in the store [or those that this shop sells].
If that is the case, I think the でafter みせ indicates the place where the action of selling take place.