Our company vBuddy.com has just created a reputation system for people who do trading on forums or by email. This way you can hold people accountable for their actions.

The concept is quite easy. Suppose you are looking to buy a phone and have found someone selling it. You just need to go to vBuddy and create a new transaction (free) using their email address. They will receive an email asking for confirmation. Once they have accepted it, the transaction becomes live and the both of you can start leaving feedback.

Once you've gathered a certain amount of reputation, you'll find it easier and easier to buy/sell online, as others see that you are trustworthy.

http://www.vbuddy.com/

Here's a quick video of how it works:
http://www.vbuddy.com/docs/tutorials/forum.htm

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Create transactions & Earn Reputation: http://www.vbuddy.com/