1. If you have any doubts whatsoever, cancel the deal. This covers doubts about the integrity of the party, doubts about the value of the trade, and any other concerns you might have. Don't finalize a trade until you are entirely comfortable with it. If the other party is overly rude, seems in a great hurry, is very unclear, etc. strongly consider backing out. Better safe than sorry.
2. A fair deal is a good deal. If it sounds too good to be true, it probably is. Never assume somebody is stupid and that you're going to take advantage of them; they are probably going to rip you. If EITHER party feels that they are not getting a fair deal, they are many times more likely to rationalize being a ripper, send late, etc. This may not have been their original intention. Again, a deal fair on both ends is a good deal.
3. Make sure to keep records of all the pertinent information. Save any correspondence you have with the other party (i.e. IMs, e-mails, snail-mail). Get their street address, home and work numbers, full name, etc. Print out hard copies in case your computer records are lost. If you want some extra evidence on your end, take a picture of your cards before sending them. You might also want to ask the other party for a scanned picture, but this is no guarantee (they could get a picture from anywhere), and not everybody has access to a scanner. Just remember: every little bit of evidence you have is another piece of protection.
4. If you want to make sure the deal goes through, take the following steps:
5. Finalize the deal before sending with a very specific list (make sure to include card condition) of what's being sent and what's being received. Print out a copy and include it in the package.
6. Be sure to adequately protect the cards you're sending. Keep in mind that the postal service can be very rough on its packages. If you're sending something of significant value, get the package insured.
7. Don't send cash through the mail. Money orders are faster than checks, but there are better records of checks and checks can be cancelled.
8. If you want to spend the extra money, send through a reputable middleman service. Several of these can be found online.
9. References increase credibility, but don't guarantee it. If they tout their references, be sure to check them all. GAB's (or similar organizations') confirmed references are a little better, but could still be fake. Remember, just because somebody traded fairly with other people doesn't mean that he or she will do so with you.
10. Ultimately, no online trade is safe. It's a risk you take. If you can't handle the risks, don't trade online. However, following these guidelines can prevent the large majority of rip-offs.