I am wondering - or - I think, making the App and front end, and, even some of the backend won't be the problem.
I discovered when working with @0foraster0 last year that Stripe (one you mentioned in the other thread) isn't available in my country, and, that to receive payment through PayPal one has to register (PayPal) with a local deposit bank. All about foreign exchange control that.
So the biggest question with an e-commerce app would be finding 1. A universal payment gateway your company as the app vendor subscribes to; or, 2. Figuring out how a person who buys and implements your software registers a gateway - as developer - does that mean you would need to cater for that as a user "setup" function?
Obviously, this can be done, but, that seems like it's the tricky bit. 2 in my last paragraph is my guess what you would intend to do....
Anyway, I hope you have good luck with the project if you start it.