"Where did you buy these skis?"
Translation:Где вы купили эти лыжи?
How about, Откуда ты купил эти лыжи? Or is где correct, or does it sound more natural?
Откуда is not good here. Где is correct, and it is exactly what's being suggested by Duo.
The pronouns ты and вы are interchangeable in this course for the lack of context from which you normally determine whether your "you" goes for one person informally (ты), one person formally (вы or more refined Вы), or a group of people (вы). Just don't forget to use plural form for the verb when using вы or Вы.
So, these versions would be equally good:
- Где ты купил эти лыжи? - asking a friend.
- Где вы купили эти лыжи? - asking a person respectfully or a group.
- Где Вы купили эти лыжи? - asking a person respectfully.
"Откуда"="where from" (or "whence" in old English). This question is about the location of some transaction, not its origin. And hence in Russian we use "где", not "откуда". This is less obvious with internet purchases, in which case English "where from" is quite appropriate, but you would still use "где" in Russian; the language predates Amazon.
Купились is reflexive. So it is either that skis bought themselves, or the skis were not the part of the deal you made with yourself on yourself.
- мыться - мыть себя - that's fine, we do that daily.
- бриться - брить себя - that's also fine, we shave our own faces most of the time.
- купиться - купить... себя? - no, that won't work. We already own ourselves, there's no point in buying ourselves, and from whom would we even if we wanted? Clearly, it's a circular reference error.
Купились would also mean that the subject "bought" (accepted, believed) some lies, and thus did a thing as foolishly pointless as buying themselves.