I have never been ripped off trading online, though I have been on Ebay (and I wouldn't have been ripped, had I been more careful).
Some tips would be to always get references, and check them out. If they don't have 5 refs that you are absolutely sure of (like above, make sure that they're not all in the same area, etc.), then make them send first. Trade only with people with tons of references until you have a bunch and can make others send first. Keep steady contact, and have them notify you when they get the cards, when they send the cards, etc.
And make sure they're serious. I once traded with a kid, and had him send first. Over the next two weeks, I sent him 4 emails, and he never responded. I finally caught him online on AOLIM and asked him where the card was. He said he changed his mind, but didn't notify me because he thought I might be mad.
Most traders are good people. There are rippers out there, but don't be afraid to trade because of a small minority. If you are careful, you should be fine.
And hurry up and unload those Pokemon cards. You're a disgrace.
J/K
Apollo