First thing you must do is determine what you want to trade. Then, after some thorough research, find a good online trading community and sign up. They usually have forums and rules of conduct, same as we do here.
Game Trader World and
Game Trading Zone are two very well know trader communities that specialize in cross-item trading.
Online Auction Exchange is also another good site you might want to visit.
The important thing on these sites (as here) is to remember to follow their rules. If you do that, everything should be ok. Get to know the other members a little before making offers. And remember that if you’re new to the online trading biz, you may be required to send your half first

.