Well you just got him/her, so give them the benefit of the doubt. Some of my new arrivals wouldn't eat for 2 weeks, others eat the day i moved them. Just leave him/her in their enclousure and feed them in there, less stress for the ball python. Also don't worry about him not eating, they can go over a year without food. Just try again in a few days in their tub.

Also don't always listen to people in pet stores. haha