Unfortunately this is rather common and many (including myself) purchase their first snake from a pet store only to find they have mites and other issues. The big concern with rescuing is you are promoting that behavior by purchasing from them and threatening to bring mites into you whole collection as they can get around.

Buying neglected animals from pet stores enables and encourages this behavior by allowing them to profit from this poor treatment. I would walk away and spend your money with people who care for them properly.