Google AdWords voucher issue

I bought an adwords voucher from a person on eBay, who has many positive feedbacks for similar items etc... I got 2 vouchers (that's how many were up for sale), however when I registered and tried to apply them they would come through as already used. I told about this to the seller, and he sent me new codes multiple times but all of them didn't work. Weird thing is, if I purposly enter an incorrect voucher of the same length as the one that the seller sent me, it'll say that it's an invalid voucher; if I enter the one that he gave me, it'll go through to the page where I have to enter my credit card info, and then it'll say that the promotional code was already used.

Am I doing something wrong here?

No, you're not. You just got ripped off.

I'm afraid what freedom said is true...

Sorry to hear about that.