I've seen shops with flies too over the years, but those are a big No No for me. I just turn back and walk away. I'd rather starve than buy what they are offering. The truth is, we're eating enough garbage already as you never know the whole story, so why buy badly handled food knowingly.
I suppose the situation is even worth in other parts of the world. It's sad as with a little attention and care these things can be avoided.