I've been very fortunate and have gotten the things I have bought in good order and the people that have made purchases from me have been nothing but pleased as far as I know. *knock on wood*
Online trading is the easiest venue to find yourself on the wrong end of an attempted (or completed) scam. You have to be careful.
There is no single way to completely eliminate the possibility but there are steps you can take to minimize the chances...
I am very carefull when it comes to dealing on the net. I will only deal with people that have refs. And usually I will only buy from those who are "oldtimers" around ca.com.
I payed some guy 20 bucks for a woofer last april, just got my refund back 3 days ago after the guy dissapeared on me and then popped up selling some gear. Made it public guy went on a rediculous rant got pwnt by the community and then sent me my cash back can be seen HERE
I think the only reason I got hit on this one is because it wasn't a huge amount of cash so I didn't investigate the guy real well. Otherwise I always talk with people before buying and feel them out, and get all of there contact info before sending them cash.