itrade will only give/receive when you do some buy/sell or trade under BST section (except Freebies). There are either positive or negative itrade, the positive itrade mean the member is good dealer.
It is the business reputation scaling in this forum, if you do business like buying or selling to some of the users they can give rating to you as a buyer
Once you do a deal with someone, go on their profile and there will be an iTrader rating for them (a number) in the top right of their profile. Click on that and it'll take you to their iTrader page, and then click 'Submit Feedback for <username>'. Make sure you use a valid thread though, as giving out fake iTrader will get you banned.
It is a good rating system in this forum. After doing a trade, and if your customer find it's good to deal with you, then he or she will leave a positive iTrader to you. This is pretty much like the feedback system in eBay.
This is one type of good reputation for you. More itrader with more reputation is good for you. Try to increase more and more itrader yourself.