I just bought this game in the sale, really looked like something I could get into. But i'm stuck. On the first level.

My supermarkets are losing money selling certain products. There is no demand for them and I have a large stock of them. I cannot for the life of me figure out how I stop buying and selling these products. I've clicked on absolutely everything. I'm stumped.

i've exactly the same problem...

I get stuck in that position too. The irony is, in the next tutorial level, they add the ability to switch items, (there are new options in the store screen). I can only guess they were trying to spare you from the game getting too complicated right away? I'm finding the tutorials to not be all that forthcoming.

Not sure if it helps: You can try switching item for other, best for normal department store are goods with quality 30+ so it will be better than whitenoise items.
Always add advertising in the center and chain it.
Make sure that your item has higher raiting or at least similar to market avarage. You can do it by spending on advertising or changing price, so it will be atractive by price.