Itrader is a little thing on the side of your posts that you can only see when you are in BST. You get an Itrader for each time you make a deal of over $5.
Check this iTrader 101 thread, for all the info. You can give itraders, to people who have successfully completed jobs, or receive itraders for jobs completed by you. itraders are valid only in the Buy, Sell & Trade section. (except the Freebies section)
Itrader is a positive/negative feedback from your Buyer/Seller/Trader after a deal over $5,good positive feedbacks can mean that you are a reputable user here on dp.
Check this out. It has the whole concept of itrader covered from a to z. That would help you. It is basically the same as feedback in ebay if you have ever used ebay. It is basically the merit of your honesty and quality.