"I saw a fashionable sweater in a store."
Translation:Я бачила модний светр у магазині.
«В крамниці» should definitely be accepted. Please use the Report button next time you get this sentence.
«В склепі» means something very different in literary Ukrainian: it's either a place for keeping coffins with dead bodies ('crypt, sepulchre, burial-vault'), or a 'warehouse'. The meaning 'shop' is dialectal and not generally used in literary Ukrainian. Please refer to the dictionary for more info: http://sum.in.ua/s/sklep
Yes, "склеп" as "крамниця" is a Polish influence in Western-Ukrainian dialect. And even in Lviv I don't hear it often, though most people would understand you if you used it (but not all - younger generation that wasn't growing up watching Polish cartoons tends to understand Polish much worse, even the words that were long used here in Ukrainian).
I also wonder where and when the warehouse sense was usually used. It's either obsolete or regional - I surely wouldn't understand a person if they used it in that meaning.
The first meaning people think of when they hear "склеп" is an underground burial vault. So it's better to use it carefully.
And "крамниця" may be even better to use for "store", because it most often is used for a small or a specialized shop, and "магазин" is for "shop" in general, big or small, specialized or not.