user74226: RBF and full blocks make 0-conf useless, because it's easy to override. Wasn't it once useful? I've heard without those it's more secure than Visa in terms of double spending. 

user35931: No. A zero-confirmation transaction is not useful for anything. Satoshi used PoW specifically, because he was trying to answer how to trust a transaction(double-spending problem).
Secondly, transaction replacement was introduced by Satoshi in the first release of the Bitcoin software, but was later removed due to denial-of-service problems.

user75001: RBF is optional and it can be seen if a particular transaction has RBF enabled or not. For transactions without RBF enabled, they are as 0-conf secure as they were previously.

74226: <p>RBF and full blocks make 0-conf useless, because it's easy to override. Wasn't it once useful? I've heard without those it's more secure than Visa in terms of double spending. </p>
 35931: <p>No. <strong>A zero-confirmation transaction is not useful for anything.</strong> Satoshi used PoW specifically, because he was trying to answer <em>how to trust a transaction</em>(double-spending problem).</p>

<p>Secondly, <strong>transaction replacement was introduced by Satoshi in the first release of the Bitcoin software</strong>, but was later removed due to denial-of-service problems.</p>
 75001: <p>RBF is optional and it can be seen if a particular transaction has RBF enabled or not. For transactions without RBF enabled, they are as 0-conf secure as they were previously.</p>
