Here's a couple rules of thumb:
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.