Just do some research into their reputation. Don't search for the lowest price. That doesn't necessarily mean the highest price is the best...Just do your homework, google whoever is selling. find their web site, find their company info, etc.
Some breeders hang around these boards and offer very helpful information, so you know they're legit. yes i'm talking about 8ball Adam. But there are plenty of others.
If it's too good to be true, it probably is. If you know the avg. price of a snake is in the thousand dollar range, and some one comes on here offering that same type of morph for a few hundred bucks, look elsewhere.