Selling brand compatible products

Hi, I want to sell mobile phone cases for that are compatible for one of the large brands.

Is this ok or will I be penalised for selling non authentic goods?

I would have thought there is no issue selling products that are compatible with brands like Apple as long as it is my own branded product just like the cases Asda sell for Iphones for example, however I have been led to believe that they would be classed as non authentic as they are not the actual Apple Brand.

Can anyone shed any light on this please I am really stuck and have stock I am worried about selling?