Hi
This is very interesting scenario in Retail, you are not going to use SAP online store (CRM) or SAP SRS but you will use third party or inhouse store system?
Although I believe SAP online store(CRM) or SAP SRS or POSDM is better option
you can study the following IDOCs
1) IDOC basic type WPUUMS01 -POS interface: Upload sales data (compressed)
2) IDOC basic type WPUWBW01- POS interface: Upload goods movements
This idoc is generated by POSDM, but you can map and generate the IDOC from your application.
Thanks
Jitesh