Do I need to registered legal business to be able to sell product?
I think that would depend on the laws in your country of residence.
If you have a local small business center, you should check with them. A lot of times it depends on how much you sell and if it is a hobby or a business. You should also look into if you need to declare your earnings. This article may be helpful to you https://www.forbes.com/sites/mikekappel/2016/12/07/when-does-a-hobby-become-a-business/#e407379449c1