How do I test my Google Wallet integration when I can't "purchase from myself" in sandbox?

I'm integrating google wallet with a php-game. I've come to the part where I want to test if my postback-script does it's job, but I can't seem to find any information on how to make a test purchase. I get this error when I try:

I'm in the sandbox so I don't understand why this is happening. Help?

Updated to encompass comments/discussion:

Create a new sandbox buyer account - aka a new Google Account/email address.

It's a dummy account so you don't provide real data nor real credit cards - re: use test cards

• accounts must be distinct - you cannot use the same sandbox Seller/Merchant account for your buyer account (that is what the error is saying).
• use a different browser (no pre-exsting/seller auth data/cookie) to test the purchase process using your buyer sandbox account (or if using Chrome open up an incognito window)

Hth.