true, it's not motion, so I guess you should consider it an exception or a special rule ;)
w + days of the week, + the word "weekend"
also, for sure w + a period of time, when you mean the time that something lasts: "Zrobię to w godzinę" (I will do it and it will take me an hour)
Well, there is no 1:1 equivalence between English and Polish tenses and aspects, although at first sight it may seem there is. The Simple aspect seems to work well even for Polish continuous verbs.
The main English sentence is "what were you buying", so perfective 'kupiłyście' is not an answer.
On its own, not taking the exercise into consideration, "Co kupiłyście...?" seems a lot more probable to me.