I agree with the previous answer that includes the screening but a better way that you can avoid pet re-sellers is through the use of an Adoption Contract or agreement. Pet re-sellers know to avoid signing these types of documents because of the potential penalties that they can incur. A real pet adopter should not have a problem with an agreement like this.
The linked document can be modified for what ever other provisions you would like to include, though you should make sure that any provisions are enforceable in your area.
If the adopter is local, one good way is to do a walk through of the living environment that the pet will be living in. You should be able to tell if the area is suited for the animal as well as maybe helping out the new owners to avoid some lessons that can be painful or expensive. If the place is uncared for then chances are the pet will be similarly uncared for, if not immediately then probably with in a few months. It is your discretion who you allow to adopt your animals, but you have to be strong enough to say no when your instincts tell you to.