I'm facing this error on debug mode but this works fine on Release mode.
error: package com.google.android.gms.ads.purchase does not exist com.google.android.gms.ads.purchase.InAppPurchaseListener AdvOSS.Droid D:\AdvOSS\AdvOSS.Droid\obj\Debug\android\src\mono\com\google\android\gms\ads\purchase\InAppPurchaseListenerImplementor.java 8
Issue solved by clearing bin and obj and rebuild the solution again.