I believe that "i ka hale" and "ma ka hale" are about the same in modern Hawaiian. Strictly speaking, "ma" is slightly more appropriate because it feels more like "at," in the same way that "i" feels more like "to."

Shouldnt "i ka hale kū'ai" also be accepted? Or is there a tiny difference i still dont know of?

why does ma take preference over i?

One of the problems with learning by just repetition without explanation is that sometimes the explanation we would be getting could be really helpful. I had made the observation that "kūʻai hele" meant "shopping." But this time the word "shop" - which I translated as "kūʻai" required the "hele." So now I'm reduced to guessing - and I'm guessing "kūʻai" is a noun and "kūʻai hele" is the verb. Can someone verify or negate this please?