Special item sets look good together and they are useful for your business as well!
All special set items boost a food category and when you bring together all pieces of a set, you get a bonus buff such as 3x tips from customers! You can buy special set items using Favors or Golden Favors which are game currencies that can be received from various actions such as catering bonus orders, celebrity level up rewards, e-Business gifts, etc.
Once you buy a set item, it goes to your inventory and you can place it from there. If you accidentally sell these items don't worry, you can buy them for FREE again later!