You don't buy card numbers individually. You buy lists of them, of varying quantity, quality and price and you just cycle the numbers until you find ones that work.
Costs per card about $5 bucks, and will get cycled into a multi-merchant hit to extract the most value from each card. A $1 Twitter charge, $50 bucks in digital gift cards, a "maybe it will work" hit for hundreds in electronics, or clothes, etc.
If nothing else, Twitter just made itself a GREAT place to test stolen numbers, since a $1 charge isn't likely to raise any flags and get the card shut down.
