An InvalidClient error typically means something went wrong with the Google Play app. Such as an account has not been set up on your device (and it must be a test account, not your developer account), or you haven’t agreed to Google’s license agreement yet, or something.
Try the following:
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Launch the Google Play app and verify that you can browse apps.
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Make sure that the account you are logged in with in Google Play is not your developer account.
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View the Android log when making an in-app purchase in your app via the Android SDK tool “adb logcat” or “ddms”. Look for any additional errors in the log.
I’ve also heard (but not sure if it’s true) that the newest version of the Google Play app is timing out with Google’s servers when making in-app purchases, but only in certain regions. In the USA, we haven’t had any problems. We’ve made several real purchases (with a real credit card) and verified that in-app purchasing is still working. We’ve posted our test results here…