When implementing in-app billing in an Android app it is a common practise to first test your purchase workflow using the static test sku android.test.purchased which is provided by Google Play.

Once you have successfully run your purchase workflow that test sku is marked as owned.

To purchase it again you have to either consume it or - if it is a non-consumable good - clear its purchase state using this command:

adb shell pm clear com.android.vending

More on testing in-app billing can be found here:
https://developer.android.com/google/play/billing/billing_testing.html