Custom sidebar is a useful feature in the Retil Market theme which allows you to add more sidebars/widget areas into the widgets page.
Adding a Custom Sidebar :
Navigate to WordPress Dashboard → Retail Market→ SideBar
Click on Add a New Sidebar and type Page Widget Area
Click on the Save Settings button.
Note: After adding the sidebar you need to attach widgets into this.
Navigate to Appearance > Widget > attach widget ti your self created side bar and assign this sidebar to any of your page.
Page With Side Customized Side Bar: