Just make deal with buyers/sellers and do honest work and increase you iTrader. If you finished deal outside Freebie section then you can exchange iTrader.
You have to click iTrader ( which in visible at BST only) of the person you want to give iTrade and then Click Submit feedback. Ask the person you had a deal completed to leave you an iTrade.
You can earn iTrader when you and a trader complete a successful transaction, but this is something manual than you give each other if you want, not something that is done automatically.
Forget i-traders.. You will get them gradually as you go. IMO reputation is more important that I-traders. I have seen people with lots of rep points and very less i-traders..
Ya, I agree that reputation is far better than iTrader and it really tells how helpful you have been on forums. But I don't know why people do not give reputations even for very nice posts. Whenever I find some good, honest or interesting post then I give reputation to the poster. Many people here do not know what is reputation and how to give reputation. Thats really bad. DP being such an active forum, people should make proper use of reputation.
You can increase your iTrader in BST section. If you are seller : Try to provide good service + Quick. If you are Buyer : Try to make Instant payment.
I'd like to say thanks to everyone who mentioned some interesting facts in this post, learnt a fair bit cheers=).