What happens if I use a fake online credit card for purchases?*
The way a credit card payment works, both in online and brick-and-mortar stores, the merchant collects your credit card details and immediately sends them to the “cloud” – aka Card Payment Value Chain, in the industry parlance – where it goes through a number of parties such as Payment Gateway, Acquirer Bank, Credit Card Network, Issuer Bank, et al.
Only when he gets a green signal – or “Authorized” message, in the industry parlance – back from the cloud will the merchant confirm your order.
If you use a fake credit card, the cloud will not give the required green signal and therefore the merchant will immediately reject your payment and ask you to continue with another credit card / funding source.
In the case of merchants who use more sophisticated tools, your payment may be rejected the moment you type in the fake credit card number i.e. without even going to the cloud.
This is not legal advice but the very act of attempting to use a fake credit card for shopping could be a crime in some jurisdictions.
*: This is the original question I answered. I’m repeating it to help me make sense of my answer in case it’s moved to / merged with some other question that I didn’t answer.