Sorting two related lists
You should probably just change how the data is stored. You should have one list which contains all of a customers data. For instance, a dictionary that follows this template:

{"name": "a", "purchased": 12}


If you had a list of those, it would be trivial to sort them, using the list sorting functions.

customer_data = [
                    {"name": "a", "purchased": 12},
                    {"name": "c", "purchased": 23},
                    {"name": "o", "purchased": 7},
                ]
customer_data.sort(key=lambda customer: customer["purchased"])
print customer_data
# [{'name': '0', 'purchased': 7}, {'name': 'a', 'purchased': 12}, {'name': 'c', 'purchased': 23}]