Hi José,
Greetings from the tip of Africa (you are almost a neighbour). My suggestion would be to create a platform in either/both Android/iOS (given the geographically area you intend functioning in - I do not think the other platforms matter much).
In this platform, (very basically) create a function that contains a web view. Remember to use the anchors in the designer when creating the page - this should take care of your responsiveness on the app. Set this aside for now.
Now go to
shopify.com. Create an online store there - the main advantage being - it will function perfectly in Facebook as well as in the WebView in your app. Once you have completed this, then copy your shops URL and return to your app and use the command WebView1.LoadUrl("YourShopsURL") and test your app. If all was done well then you should have a complete eCommerce system in your app that is fully responsive - BUT - that is also supported on a website and Facebook.
I trust this will give you some pointers. If you need anymore information, this forum is full of good people who are more than willing to share their experience with you - just ask.
Finally, "Give a man a fish and he will eat for a day - teach a man to fish and he will eat everyday !!"