Have you built the Oil Mill, the Snacks Factory, or the Pasta Factory? If you are producing those goods but don't know where to sell them, you need to check the pending upgrades from time to time! You only need to tap on the hammer icon to see what areas can be expanded and built in the supermarket and the pending requirements.

If you feel stuck with the products mentioned above, you probably need to build the grocery shelf. Remember that you need to upgrade your supermarket level to unlock new sections. Once you reach the desired level, you can new shelves.