If Etsy wants to moderate this, they can enforce GTIN verification. Google Shopping does this so they know how to rank bids for the same items by multiple department stores. Google Shopping requires that you put in some GTIN value (they verify it) or you make up your own Manufacturer Identifier (either you control manufacturing or you have a virtual item). The downside of using your own identifier is that they penalize you with worse cost / rankings but thats the cost of not having a GTIN.