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This issue is being caused by not being logged into your developer central account at the same time you are trying to use the test sandbox accounts.
Or may be you are using two different developer accounts for seller and buyer account.  Please only one developer account to create buyer and seller.
If i log into my own developer central account and attempt to use your sandbox buyer account, i get the same error.  As soon as i use a sandbox account i have setup, it works fine.