When is an "in app purchase" not an in app purchase? 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?
I think that you have mis-interpreted


  For applications and all in-app products that you choose to sell on
  Google Play


to mean all purchases made through apps distributed via Google Play rather than purchases made using the Google Play Store. When you use the Ebay app they don't take payment via Google Play and therefore do not need to pay Google 30%.

Having said that... as mentioned by Dan Hulme the Google Play terms prohibit using other payment mechanisms for purchases that are are entirely within the app (i.e. excluding magazine subscriptions, other kinds of downloadable content, and purchases of physical goods)


  App purchases:
  
  
  Developers charging for apps and downloads from Google Play must do so by using Google Play's payment system.
  
  
  In-app purchases:
  
  
  Developers offering virtual goods or currencies within a game downloaded from Google Play must use Google Play's in-app billing
  service as the method of payment.
  Developers offering additional content, services or functionality within another category of app downloaded from Google Play must use
  Google Play's in-app billing service as the method of payment, except:
  
  
  where payment is primarily for physical goods or services (e.g., buying movie tickets, or buying a publication where the price also
  includes a hard copy subscription); or
  where payment is for digital content or goods that may be consumed outside of the app itself (e.g., buying songs that can be played on
  other music players).