I've been fortunate with the folks on BladeForums. I look at their feedback [Click their name, view Public Profile, and the default tab (Feedback) has both a summary and a "View all Feedback for xxx" link -- NOTE: clicking on the View all feedback link takes you to where you go to leave feedback]. I also look at their number of posts - what I'm looking for is that they've established themselves. My belief, naive though it may be, is that most people on BladeForums as well as here at the Yard, consider themselves to part of a community and are therefore concerned with preserving their reputation.
For that reason, I've often paid with money orders and have engaged in commerce with folks here where trust was involved. On BladeForums, if your deal really goes bad, there's also the "Good, Bad, and Ugly" sub-forum where you can post about a bad experience. However, I'd strongly recommend communication and a certain amount of understanding in transaction where the seller is an individual -- you're dealing with folks who may have an injury or emergency that knocks them out of touch and you'll end up looking bad if you jump on somebody who was in the hospital. If I catch somebody in a lie, I'm pretty unforgiving, but otherwise I try to give them the benefit of the doubt.
I wholeheartedly agree with Bruce as far as eBay is concerned, but I'm a bit less security conscious on the forums, and it hasn't bit me yet. However, you should go with your gut, if you don't feel comfortable with a deal, then it's probably best to wait for the next one.