I have discovered that asking the person selling the animal several questions makes a big difference in weather I buy from them. If they can't answer the fallowing questions, I don't buy no matter how much I want the animal.
1. Feeding schedule and size.
2. Weight and length.
3. Attitude. (Can tell who is honest by their reaction usually).
4. Ask about the parents genetics.
5. In person I always handle them for more than 5 minuets. (sometimes longer, to see if the animal relaxes or remains tense or becomes defensive.)
Not foolproof but it is a good practice.