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.
When a customer returns an item for reason or another you need to handle and record such incomplete sales properly. Sales return form is designed to accomplish such crucial business tasks that help you improve your sales. Each sales return when recorded in the system gets a unique ID. You have to relate your returns against specific transactions and invoice number.
At times you receive lot of replacement claims from your customers that you need to manage accordingly. Inventory Sol Replacement form inside Sale module can handle this effectively for you. This form is used to handle replacements against items that are sold over the counter not online. To record the replacements accordingly item can be tracked through item serial number.