Well, first thing to do is make sure the source is trustworthy and has a good selling rep! I've heard tons of horror stories, that one included (which does suck big time. Sorry to hear that Becky), about shady deals.

Next would be to make sure you can get extra pictures and size indications if possible. That way you know the snake you want is really what you're seeing. This is where my minor bad story comes in: I found a beauty of a female Hypo Bloodred corn snake on SerpenCo.com, a very reputable business out of Florida I believe. He couldn't give me any other pictures due to work load, so I ordered her anyway because she looked like just what I wanted. The fault was mine in not asking him via email how BIG she was... I expected a snake around 50g at least and got one that was 6g. Turns out she was (and still is) a slow grower and was the size of a hatchling. Oh well, she's a great little snake...now I just have to see about getting her to grow up.

Payment is the next thing to be sure of. PayPal is pretty good but can still have its drawbacks. Get whatever deal made on method of payment in writing (paper or via email) whenever possible.

Shipping is the most nerve wracking part of ordering any critter from far away. That's the one part where neither you nor the sender have control. Make sure to get a tracking number and keep your eye on the package.

So, mostly it's all about making sure the source is a good one and that you keep track and records of all parts of the transaction. Nothing a little research can't straighten out.