From what I've found communication is key. I haven't bought anything off this site in particular, but I have a few others just ask questions, the op more than likely doesn't have anything to hide. And if they don't answer your questions it wasn't the board for you.
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1) Trust no-one. What I mean by this is, no matter how active on the forums, how good the communication is, etc. Always use paypal with protection (don't click personal transaction, I made that mistake)
2) If you're really worried, deal in person and with cash.
3) Take the time to carefully check the equipment over, or ask for LOTS of pictures if buying through mail. Anybody that's not willing to allow you to take the time could be trying to swindle you.
I second poutanens suggestions. I've bought and sold stuff from fellow forum members a few times and it worked out well on each occasion. There was lots of communication back and forth before any of the deals were done and we used PayPal for each transaction. I would recommend that if the deal seems too good to be true and you get any weird vibes while conversing back and forth to just walk away.