It’s not a shopping app but if you haven’t already gone through it, I would recommend following this learning path available in Mendix Academy, It’s a complete walkthrough to develop an application that will help you later to develop any application of your use case. 
https://academy.mendix.com/link/paths/31/Become-a-Rapid-Developer

There are also few webinars which you can browse
https://academy.mendix.com/link/webinars

Mendix youtube channel also has good content. You may have a look there too