I'm not quite sure if this one will work or not, but today (August 1st in the US) is the last day Blizzcon and WW1 keys will be able to be registered for the beta. Majority of the people buying these keys are not going to know this, since there isn't a really clear post about the keys being invalid after the 1st of August.
If you have a key, I would think you could probably make some quick money by selling these keys on a 1 day auction, buy it now, on eBay, probably being able to sell the keys multiple times.
Just make sure the key is not sent out until tomorrow. Do not send the key through email, but send the card in the mail, or better yet, tell them you are photocopying the card for security purposes, and send them the photocopy in the mail. Meanwhile photocopy the card as many times as you need and sell it as many times as you want. Since these cards will be invalid anyway after tomorrow, the people purchasing them won't get a "card # already in use" error, which would not implicate you in any sort of scam. It would be their own fault for not finding out the beta keys are not valid after the first, and I wouldn't think you would have to give back the money, since you sold a valid key at the time.