The standard answer usually given is that, as stated in the terms, the commission is made only if the buyer is new to GCU.  
 
Here's a way to verify this: Find a friend/relative who has not ordered your cards before and, thus, does not have a GCU account. Offer to reimburse this person for a card purchase. To control as many variables as possible, be with them when they click your web/blog/email advertising link.  Have them immediately click to purchase the card (without looking at any other cards on the site), go through the sign up process, and finalize the purchase. Then wait and see if you get a seller notification! Let us know. I've planned to do this myself, but just haven't done it yet. If several of us try it, it will be a good test!  
 
An added bonus to this is that your friend/relative might decide to keep the account and buy cards in the future!