Wow, that is a great idea if you are going to create it successfully it would be a pride for you because you had made a  successful e-commerce but I think that it would be easy for you to create a shopping cart without third party, creating an e-commerce is really hard and it takes so many process before you have made it. You should have strong and enough knowledge about e-commerce and App development before you create that one.

Anyway, if you had made it successfully, kindly post it here so we can visit it. Thanks