I followed exactly these instructions but the transactions keep on succeeding on the new checkout. I even tried to meddle with the buyer account to remove all fundings but the UI doesn't allow me to remove either account nor credit card (which was recommended in another thread). More over, when creating a new buyer test account, it forces you to have a bank account, a discover card or paypal card so there is no way to start off with an account without funding. PayPal is being so inconsistent and unsatisfactory in whatever they do.
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