Now I got the following response within the log :
Starting async operation: launchPurchaseFlow
Constructing buy intent for android.test.purchased, item type: inapp
Unable to buy item, Error response: 7:Item Already Owned
Unable to buy item (response: 7:Item Already Owned)
The only info that I got from _PurchaseCompleted event is the Success = false.
How to catch the error response code within the _PurchaseCompleted event ?