Web Services for POS

We are thinking build native apps ( Android and iOS ) for POS with Web Services. Where I can find such APIs OR we have to write Web Services First?

Please guide.

For offline you could look at this -
There is already a proposed project for POS on frappeJS

for API

But I did not get any ready made APIs, I think I have to write APIs from scratch.

You can see in network tab of browser.

Nice suggestion, there we have overhead, lot of attributes unable to understand the meaning and use of that.

You can go in erpnext repo. you will find there. whitelisted methods.