Buy unique GTIN barcodes from a bulk supplier. GTINs are barcodes (also called EAN, UPC, ISBN) which are used by Google Shopping to identify products.
In most cases these codes are required and will get your products approved. In other cases it will improve performance on Google Shopping.
NOTE: Some market places (Amazon & Bol) require merchants to only have brand registered GTINs, so these GTINs will not work. Please test with a free/few GTINs first.