A stock-keeping unit (SKU) is an identification code depicted as a machine-readable bar code for a store or catalog's product.
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Further edit. Ideally there needs to be tight governance around product master data for Ecom sites and most decent retailers curate it carefully.
For market places however since anything goes, sellers get to create their own sku's (and Amazon gets paid every time) plus sellers can promote their own sku than trying to fight with other sellers for the buy box in a single listing. All of this ends up with listing spam on Amazon which makes for a crap user experience, but as long as Amazon gets money, they'll look the other way