"This food is on sale."
Translation:이 식료품은 판매 중입니다.
식료품 is, via Hanja, counted food products, aka groceries.
noun+ 중이다 is a little easier than the usual use of 중. Normally it's with a verb, 하는 중이야 = I'm in the middle of doing it. Using it with a verb means it's in progress (and is practically the same as 하고 있다 but sounds cooler). Using it with a noun is often done in fragments, like a sign that says 시험 중 (test in progress). You can simply add 이다 after 중 to make it a complete sentence. 시험 중입니다 (there is a test in progress).