買う, durative or instantaneous?

Question

I'm at the point in my textbook where it explains the use of -ている and the three kind of verbs related to its use: stative, durative, instantaneous. The book says I should ask myself if saying something such as 昨日は１時間買っている is logical. At first view, it seems it isn't, and 買う is an instantaneous verb (buying something, which is exchanging money for something, over an hour, seems a bit ridiculous). But thinking about it a bit more, this situation came to my mind:

A: あなたはどこですか。
B: 食べ物を買っている。

In this context, it does seem possible he's saying 'I'm buying food'. So, my question is: is there a fool-proof way to know if a verb is durative or instantaneous? Some list or tool around I can use to verify what type of verb each is?

Answer 1

I would say that 買う itself is instantaneous, unless it's a rare situation where your job is like a purchasing manager or something, where you're just constantly buying things. Even then, it still seems questionable to say it's durative. You would also use 買っている for habitual buying actions.

For your example, it'd be better translated as "I'm out buying food", for which you could say

A: あなたはどこですか。
B: (スーパに)食べ物を買いに行っています。

In this situation, the 行っている is stative ("being 'gone to'/at someplace"), and the 買いに is the purpose of you're going (〜＋に行く).