Sale Invoice
Here you can create invoices against the items you sell to your customers, each time you create a new invoice a unique invoice number is created .
Customers ID: For existing customers you can pull up the customer ID together with address and other relevant information. In case you have new customer either you may assign proper customer ID or if you believe the customer is one time then you may treat that as a guest customer without assigning any ID.
Transaction ID: In case payment mode is other than cash like PayPal or Bank then you have the facility to mention the transaction ID.