This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...dels which cannot. Retail is the most obvious example. Customers need to be able to pay the cashier and leave the store instantly. They can’t be required to wait around for 10 minutes or longe
r for their transaction to confirm. If retailers are unable to mitigate the risk of fraud when accepting unconfirmed transactions, then they simply wont accept Bitcoin.
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w to double spend
Double spending an unconfirmed Bitcoin transaction is actually rather easy for people with the technical ability. The easiest way is to create two transactions...