I also think you did fine.
I probably also would have mentioned to the customer that big box pet store babies are often stressed and nippy, but I have well-started non-nippy ball pythons available and my email address is ...And in the meantime let's go check out the supplies and I'll show you the correct stuff to buy for a ball python...
BTW I wouldn't normally get between a vendor and a customer, especially on the store's property, but in this case the vendor already lost the sale.