GC Zen buys GC purchased from eBay all the time. When GC Zen said they didn't accept card bought on ebay, that was just a bogus excuse to not buy any more of whatever brand of card you sent them, because the bloggers have been banging the drum extolling the virtues of somehow getting rich reselling GCs from eBay, and then the market was flooded with that brand of card. You probably got in too late.

On the other hand, I'm pretty sure they warn you not to send them anything in the mail until you get the email from them confirming that they will accept your order.