There is never a guarantee, but the first thing to do is to try to work with someone who has a little of positive history. It won't take hanging around very long to figure who some of the "good guys" are. That doesn't mean that folks with low iTrader numbers, etc, are not good to deal with, you just need to try to have references, etc. A lot would also have to do with the value of the cue deal. I would be reluctant to do a real high end trade with a new poster, etc. In these cases, you can often find a mutual person to broker the deal for you.
There are plenty of folks on here that I would send a cue on their word alone. I know without a doubt that they would do exactly what they said they would do. I also know a couple that I would not want to deal with at all. The gray area between those are where you have to feel comfortable.
The transactions themselves are simple. We agree on a trade tonight, and we both ship out tomorrow via an agreed upon method.
Joe