The Play Store policy on in app transaction fees reads like this:
For applications and all in-app products that you choose to sell on Google Play, the transaction fee is equivalent to 30% of the price. You receive 70% of the payment and the remaining 30% goes to the distribution partner and operating fees. (Reference: https://support.google.com/googleplay/android-developer/answer/112622)
I get that this applies to things like premium currency, extra levels, even e-magazine subscriptions. However, there are some cases where I assume, perhaps incorrectly, that that a particular app couldn't possibly be paying 30% on purchased items. To name just one example of this, eBay. I doubt they pay 30% of the price of an item when they themselves collect maybe 20% of the sale price, but correct me if I am wrong.
Is there a rule, hopefully officially written down, which explains when a purchase made through an app is not subject to the 30% fee?