I want to know if it is possible to automatically reset the product quantities in shopping lists after customer places the order?
Not the order, but the shopping list itself.
Eg. Change from 5 back to 1?
You can implement it as a part of the custom checkout workflow based on one of the default workflows. For more details the documentation.
For example, that change can be a workflow action described in the last workflow transition definition and executed when the order is created.