iTrader is like your feedback on ebay. People can leave you iTrader points if you have done a transaction of some sort with them (Buy/Sell/Trade)
Whenever you do business in Buy/Sell section, after completing the project, you ask you buyer/seller to leave positive iTrade if it was successful then the rating will increase. So this will help you to gain more business base on your iTrade rating. This is trusted rating to do business.
Itrader is very important, it's kind of trust creating factor. It shows that you have dealt with many persons in the past and everyone were good and satisfied