Each item that you send into Amazon must have a scannable bar code. If you are sending items in co-mingled than this may be nothing more than the original UPC. All other products will need a bar code label. Here is what you need to know:
1) Make sure you label each product
2) Make sure all other bar codes on the product are covered
3) Make sure the bar code is able to be scanned (don’t put the label around corners of a product)
If you are using a 3rd party software you will be able to print labels as you go when listing. To do this you will need a Dymo label printer.