user1561: I keep hearing people say that 0 confirmations are safe at small dollar amounts because it would take more money to do a double spend hack. At what value does it start to become unsafe to trust a 0 confirmation transaction?

user575: There is no hard and fast rule. It depends on who is trying to attack and how. But I'd say that for amounts of up to $30, 0-confirmation transactions are usually very safe. For larger amounts it depends on the exact situation. Such transactions are safer if the good offered is illiquid, if the transaction is done in person and if the sender is identified in some way.

user85: The easiest way to do a double spend against someone who accepts zero-confirmation transactions is with a Finney attack. To perform a Finney attack, you must first mine a block and be willing to risk losing that block. So the answer is that if the value of the transaction is small relative to the risk of losing a block reward, you don't have to worry about the attack.
Using today's numbers: An attack with a one minute window has about a 10% chance of costing you a block. The block reward is 25 Bitcoins and a Bitcoin is around $600. So for amounts much less than $1,500, you don't need to worry about a double spend. Nobody's going to risk losing $15,000 to steal $1,500.
Note that this assumes you know how to accept zero confirmation transactions! You must watch at multiple points in the network and ensure the fee is adequate.

1561: <p>I keep hearing people say that 0 confirmations are safe at small dollar amounts because it would take more money to do a double spend hack. At what value does it start to become unsafe to trust a 0 confirmation transaction?</p>
 575: <p>There is no hard and fast rule. It depends on who is trying to attack and how. But I'd say that for amounts of up to $30, 0-confirmation transactions are usually very safe. For larger amounts it depends on the exact situation. Such transactions are safer if the good offered is illiquid, if the transaction is done in person and if the sender is identified in some way.</p>
 85: <p>The easiest way to do a double spend against someone who accepts zero-confirmation transactions is with a Finney attack. To perform a Finney attack, you must first mine a block and be willing to risk losing that block. So the answer is that if the value of the transaction is small relative to the risk of losing a block reward, you don't have to worry about the attack.</p>

<p>Using today's numbers: An attack with a one minute window has about a 10% chance of costing you a block. The block reward is 25 Bitcoins and a Bitcoin is around $600. So for amounts much less than $1,500, you don't need to worry about a double spend. Nobody's going to risk losing $15,000 to steal $1,500.</p>

<p>Note that this assumes you know how to accept zero confirmation transactions! You must watch at multiple points in the network and ensure the fee is adequate.</p>
