Agree, but not for your reason. Although, before I looked it up, I would have.
Learning for a lifetime. :) http://www.oxforddictionaries.com/words/can-or-may
I think "Where may I buy a scarf?" should be accepted. The mouse-over tips for "можна" give both "one can buy" and "one may buy" as hints. Sure, splitting hairs, one can argue that in English there is a distinction between permission and capability, but I think the de facto difference is merely in politeness level.