user3176: As a regular user of Bitcoin, I often send bitcoins to other, or receive BTC from others, sometimes this might be a trade or deposit. Some traders require at least 6 confirmation, some require at least 3.
I want to how many confirmation is enough to ensure the transaction is successful?

user3327: The lesson of the recent v0.7/v0.8 fork issue is that simply waiting for 6 or more confirmations is insufficient.  You also need to satisfy yourself that there are no competing blockchain forks
e.g. this thread discusses a double spend proof-of-concept that was executed during the chain fork: https://bitcointalk.org/index.php?topic=152348.0

user153: It depends on your risk model.
If you can trust the person paying you, you can accept payment on 0/unconfirmed if you want.
As a merchant or trader, you want to use the configuration that is more secure (no incoming connections permitted, explicity connect to well-connected nodes).
With zero confirmations you are vulnerable to the race attack and the Finney attack, as well as the 51% attack.
With one confirmation you are vulnerable to the 51% attack.  There could also be a miner with a lot of hashing power who could get a couple blocks in a row, so three confirmations removes most of them.
With six confirmations it is essentially mathematically impossible for an attacker with less than 51% of all mining capacity to get six blocks in a row. and still surpass the longest block chain.  With 51% or a lot more than 51% the attacker can get six confirmations by creating a parallel blockchain in which only transactions approved by the attacker get included in blocks.
A merchant like a retailer can likely accept even on 0/unconfirmed in most instances (e.g., purchases up to $100 worth).     A merchant that ships e-commerce might hold off until the transaction has three confirmations.
A cash, face-to-face cash trade will probably be best if three confirmations for a large amount, maybe one confirmation for small amounts. 
Any recommendation would need to come only after determining which risks are deemed acceptible.

3176: <p>As a regular user of Bitcoin, I often send bitcoins to other, or receive BTC from others, sometimes this might be a trade or deposit. Some traders require at least 6 confirmation, some require at least 3.</p>

<p>I want to how many confirmation is enough to ensure the transaction is successful?</p>
 3327: <p>The lesson of the recent v0.7/v0.8 fork issue is that simply waiting for 6 or more confirmations is insufficient.  You also need to satisfy yourself that there are no competing blockchain forks</p>

<p>e.g. this thread discusses a double spend proof-of-concept that was executed during the chain fork: <a href="https://bitcointalk.org/index.php?topic=152348.0" rel="nofollow">https://bitcointalk.org/index.php?topic=152348.0</a></p>
 153: <p>It depends on your risk model.</p>

<p>If you can trust the person paying you, you can accept payment on 0/unconfirmed if you want.</p>

<p>As a merchant or trader, you want to use the configuration that is more secure (no incoming connections permitted, explicity connect to well-connected nodes).</p>

<p>With zero confirmations you are vulnerable to the race attack and the Finney attack, as well as the 51% attack.</p>

<p>With one confirmation you are vulnerable to the 51% attack.  There could also be a miner with a lot of hashing power who could get a couple blocks in a row, so three confirmations removes most of them.</p>

<p>With six confirmations it is essentially mathematically impossible for an attacker with less than 51% of all mining capacity to get six blocks in a row. and still surpass the longest block chain.  With 51% or a lot more than 51% the attacker can get six confirmations by creating a parallel blockchain in which only transactions approved by the attacker get included in blocks.</p>

<p>A merchant like a retailer can likely accept even on 0/unconfirmed in most instances (e.g., purchases up to $100 worth).     A merchant that ships e-commerce might hold off until the transaction has three confirmations.</p>

<p>A cash, face-to-face cash trade will probably be best if three confirmations for a large amount, maybe one confirmation for small amounts. </p>

<p>Any recommendation would need to come only after determining which risks are deemed acceptible.</p>
