Companies creating for internal use, or in this case for external/partner use, APIs that are asynchronous/event based to make them realtime is becoming more & more popular - this is just one public example. Great that EBay would share their experience.
This type of use case is why we built an Event Portal feature coming soon called Event API Products . So watch for that to be released in PS+ Cloud in the next few weeks.
If you don’t have a PS+ Cloud trial account - go get one 