This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...ren’t, but they don’t need to be to serve the purpose.
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Given double spend detection, it becomes very difficult to successfully defraud a retail
er. Ideally what you would like to do is broadcast the valid payment, leave the store with the merchandise, then broadcast the double spend. The problem you have is every miner (from the very be
ginning of Bitcoin through today) has been programmed to accept the first transaction it sees as valid and reject subsequent double spends. If you wait until you are in your car driving away wit...