Different sections of the store usually have different people in charge of them. Ask to speak with the person in charge of the reptile section.
Ask what they are doing to help that emaciated ball python. Suggest that they remove the animal from display, because they shouldn't sell an unhealthy animal to someone.
Come back the next day. If it's not done, ask to speak with the manager. Repeat the process.
If it's still there the day after that, contact the corporate office, and ask them to do something about it.
Different States have different laws regarding the condition of animals on display or for sale. Find out what your State's laws are, so you know whether you can report the store if it becomes necessary to do so. Try to speak to the person who is responsible for investigating cruelty or neglect cases against pet stores.
If this all sounds like a lot of work....it is. But no one else is going to do it, if you don't.
If we all did this more often, there would be a lot more knowledgeable people in charge of the reptile sections at these stores, and a lot fewer sick animals put up for sale.
In most of these instances, the employees at the store are NOT following the corporate guidelines for caring for those animals. This needs to be reported to corporate when it happens, if the manager won't correct it.
This is not a situation of "PetCo is bad". They're not. But some of their employees are, and they can't correct that if they don't know about it.