Unfortunately, as an AI language model, I am not able to create diagrams. However, I can provide a list of potential use cases for the system based on the information provided:

1. Customer purchases an item in-store using cash or credit card
2. Customer places an item on lay-by and makes a deposit
3. Customer makes part-payments on a lay-by item
4. Customer pays off a lay-by item in full
5. Customer fails to pay off a lay-by item and it is restocked
6. Customer returns an item for warranty repair
7. Staff member enters new stock item into the system
8. Staff member generates and prints a barcode for a new stock item
9. Staff member generates a report of stock on-hand, in the warehouse, on order, or sold
10. Staff member generates a report of stock location
11. Customer checks their loyalty programme grade and discount percentage
12. Customer requests a price for an item
13. Customer pays an invoice online
14. Customer links to the websites of musical instrument wholesalers and manufacturers
15. Stock item is sold and removed from the system
16. Staff member scans a barcode to add or remove stock from the system.