I think it means they're stolen cards, to be honest. If you look at the cards, there are two types. There's the loose card that says "not valid until activated at the register" and there's the card that comes in a bubble back or in a Live kit, these don't have that wording. I believe the ones on eBay that say "new account only" are stolen cards that require activation (and obviously can't be activated). Because a new account almost always gets that free month I think they expect that you won't notice a problem until it's too late.