1_他买什么？ = What does he buy? 他在买什么呢？ = What is he buying? 2- 我去商店。 = I go to the shop. 我在去商店呢。 = I'm on my way to the shop.

1_他买什么？(what is he buying.the point is "what") 他在买什么呢？(by this way to ask,the point is not "what",this question is more like:what is he doing. 2- 我去商店。(the answer of"where are you going to? ) 我在去商店呢。(more like the answer of"where are you?"you tell them i am on the way to a store.

我在[去商店的路上]。or 我[要]去商店。makes more sense than 在去