You will build a webpage, Joomla prefered. I will send you a website which I want mine to look like and you will build the exact same website.
You will build android app and ios app to go with the website. (I will send you the app I want you to make, you will download it and see its functions and make same for me)
You must have knowledge on ebay api and amazon api.
Checkout process will have two options
1. Cash at office (Order placed pending confirmed payment - admin manually confirms the payment)
2. Mobile money (User will basically enter a mobile money ID code and if there is a match with the codes stored in our database, then order is placed.)