[corporatesans]

The terms around IAP state that "content, functionality, and services" used in an app must be purchased using IAP.

Further, you may not complete a checkout process by linking to an external site.

Critically, if an app uses IAP to purchase goods or services to be used or consumed outside the app, it will be rejected.

Bottom line if you want to sell socks, sandwiches, or massages for USD in an app, you can, but your checkout process must be native.

Competition with IAP is not at issue here.