C# The type or namespace name `List' could not be found. But I'm importing System.Collections.Generic;

The issue comes from your instantiation of new List(). These also need the generic component:

public static List<string> items = new List<string>();
public static List<double> itemsprice = new List<double>();
public static List<double> qu = new List<double>();

That is, there is no type List but there is a generic type List<T>.

More information and examples of instantiating the generic List<T> can be found in the MSDN documentation.