These are virtual credit cards of the mastercard variety. There are also Visa versions of these, but not on this sales thread.
They're very useful for unlocking free tirals that require you to give the service your credit card information. So for only $0.25 you unlock as many free trials from different services as you want (if you want another trial from the same service you will have to buy another virtual credit card, of course). These are perfectly legal, and they don't contain any real money.
They're in the following format:
card number, expiry date, security code, type of card (all Mastercard)
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