A brand kit(opens in a new tab or window) is a collection of design choices and 
assets that make up your company’s personality and 
its look and feel.  brand kit  allows you to select brand fonts,
brand colors, and brand imagery, like logos and feature photos.
that you want everyone at your company to be able to use — and add them all to your design platform.

how to sell brand kit?
you can sell brand kits through many ways some of them are given below
linkedin 
facebook
instagram
fiverr
upwork

how to make brand kit? 
if you don't know photoshop , just go to canva and search brand kit 
it will show you many templates and you can edit the template and earn money easily.

if the client gives you image , that you have to use this colour , simple upload image on canva and it will
show you it's used colour