«Я можу» sounds a tad less natural to my ear. «Мо́жна» is impersonal, while «я мо́жу» is a personal verb, it shifts the meaning to 'you' a bit: as if you can buy a scarf in a your own places, or in your own way, compared to others.

But the difference is very subtle, and I think both should be accepted.

• "можна" does not refer to anyone and is best translated as "may (I, you, he, she, etc.). It is commonly used to ask permission. e.g. "May I go to the bathroom" - "Можна заходити до туалету?"

• "я можу" refers to you and is best translated as "can". It stresses that you can do something, not John, not Mary, not Paul, etc. It is also commonly used to express an ability to do something e.g. "Чи можу я йти до туалету?" (I dunno, can you?)

«Шарф» means 'scarf', «хустка» or «хустина» mean a 'kerchief'. (At least in literary Ukrainian. I don't know much about Ukrainian dialects. This course teaches literary Ukrainian.)

Please refer to my answer here for more info: https://www.duolingo.com/comment/15646521$comment_id=18846464