Here are a few suggestions, not meant to be exhaustive:
1. make sure you see a picture of the gecko
2. ask for info about the gecko's diet, and especially supplementation. Ask anything else you want and don't buy if you don't get prompt, polite, accurate responses.
3. ask how the gecko will be packaged (unless you're buying from a large, professional breeder, although some people will say you should ask then, too). Read up on the best way to package a gecko, so you can evaluate the response
4. Very important: check the seller on the BOI (board of inquiry) at
FaunaClassifieds Home Page. That way you will know if other people have had trouble with this seller. You still have to evaluate for yourself the info that you read there, but it will give you an idea about some of the issues
5. Check to see whether the seller is a minor. That doesn't mean you shouldn't buy from minors, but you may need to do a little research to find out what could happen if this person doesn't come through. I have had much good experience and one bad experience buying from a minor.
6. Probably the safest ways to pay are credit card or paypal, although they are not foolproof. There are some people who are not set up for either and will request a money order or check. This is more risky because they could "take the money and run", but in certain circumstances will work out. Personally I have generally had positive experiences except for one situation (paypal) where I had to fight to get my money back from the seller who didn't deliver (and was a minor) and another one where I actually had to go out of state to take someone to small claims court (I won).
I hope this is helpful
Aliza