This class has two constructors. The default constructor (the one that takes no arguments) should call the various set methods to initialize the first and last names to "None", the seller ID to "ZZZ000", and the sales total to 0.
The other constructor for the class should initialize the data members using the passed in arguments. It takes 4 arguments: a character array with a Seller's first name, a character array with a Seller's last name, a character array with a Seller's id number, and a double that holds the Seller's sales total. As with the previous constructor, the data members should be initialized by calling the various set methods.
means C++ programs then I am surprised that you haven't written any constructors/functions yet.
Also, in this case the program given in the description itself is almost complete you just have to add bodies to the functions as described.