Subject: Customers, Sellers and Books

Hello Barry,
 
I have been assigned a project to create a Microsoft Access database which will be based on a company which sells books online. 

I obviously first need to create an entity relationship diagram but I am struggling as to how to determine a customer. 

This is because as the brief states "the company specialises in remaindered and second-hand textbooks. 

The company buys surplus books from publishers but also buys individual books from students. 

The books are advertised and sold via the company's website." 

The system I need to create will need to record the basics of recording sales but will also need to produce invoices etc. 

However I am struggling in how to define a customer as a student could also be a customer which sells a book to the company ? 

And so obviously I cannot have two tables one for students and one for customers as this runs the possibility of duplicating the data.
 
Any help will be greatly appreciated.
 
Kind regards,
 
Michael