I am an intermediate-level student, studying in Japan. All teachers are native Japanese speakers.
I was asked to write an example sentence using "ところ", using it to express a temporal concept such as "about to," "while," etc.
I decided on the following sentence, in English:
When I was about to buy a manga, I noticed that it was the wrong volume.
To me, this is somewhat abstract, but clear enough: either in a physical store, or online, I was about to buy something when I realized I had made a mistake.
In Japanese, I expressed this as:
This was so incomprehensible to my teacher that she came over to my desk urgently during class to explain how I was using the wrong kind of ところ (I'm not sure what she was getting at as she later erased her initial notes). I'm aware that there are secondary grammatical errors here, but please read on.
The only way I was able to explain it to her was to physically act out and narrate the act of walking up to a register, looking at a book, and realizing a mistake. This was still so surprising to her somehow that she took until the next day to come back with this rewritten sentence for me:
In English, I read this as:
When I had just gotten in line to buy a manga, I noticed that it was a different volume from what I wanted.
This is certainly much more concrete and detailed, but what is it about my initial construction, 漫画を買うところ, that was so confusing to a native speaker? How would my original idea, "about to buy a manga," be expressed in Japanese, if not this way?