I'm a regular there, been a member for over six years, four gold star trader (trades with 200+ unique members). The vast majority of my dealings there have been good, obviously, or I wouldn't have kept using the site this long. I have had a few deals go bad, even one in which a double gold star trader stole a N64 bundle worth over $100 from me which I sent first when I was new to the site. However, the advice above will generally keep you out of trouble there. If you aren't willing to send first to establish a good feedback history, you may as well stay away from the site. It also makes no sense to try to trade with other low-feedback members so you don't have to send first, as you're much more likely to get scammed than if you trade with established members at the outset.
The only other thing I'll say about GameTZ is that, from my experience, it's a better place to trade than it is to sell items, if you want to make as much as you can. I guess the name implies that. Conversely, you can find some good buys there. Overall, it's a very good site if you're willing to trade within the framework of the feedback system, and have the patience to trade with established traders until you build up enough feedback where you can stipulate that less-established traders send first to you.