Sheesh. Now you tell me.
I can't think of a single time I've been burned on a deal at CAG... you guys have been remarkably consistent. I haven't needed to ask any sellers to ship or email first... hell, sometimes they'll do it without me even asking, even though they've got a lot more feedback.
Then again, I choose my transactions carefully. If someone just randomly appears offering $50 of PSN credit for $40, and has only a couple of trades to his name, that's a pretty clear sign that they're going to screw me over in the trade. I guess the moral of the story is be cheap, but also be cautious, and don't get greedy. If it looks too good to be true, all you're going to get out of that deal is a hot beef injection.