These get confused constantly because some identifiers are universal (you obtain them) and some are platform-assigned (the platform creates them for you). Getting this straight before you touch a listing form saves you from the single most common new-seller catalog mistake: buying the wrong kind of barcode, or assuming an identifier you were handed by a marketplace is something you can reuse elsewhere.

Identifiers you obtain

  • UPC / EAN / GTIN — universal barcodes that identify a specific product globally, obtained through GS1 (the standards body) or, less reliably, resold by third parties. Most marketplaces require a valid, GS1-registered identifier to create a new listing, and increasingly verify this rather than accepting any 12-digit number.
  • GTIN (Global Trade Item Number) is the umbrella term — UPC-A (12 digits, common in North America) and EAN-13 (13 digits, common internationally) are both specific formats of a GTIN. A marketplace field labeled "GTIN" will generally accept either format; a field labeled specifically "UPC" may reject an EAN-13 or want it reformatted.
  • MPN (Manufacturer Part Number) — a manufacturer's own catalog number for a part or product, distinct from a UPC/GTIN. Some categories (especially parts, electronics accessories) ask for both a GTIN and an MPN, or accept an MPN as an alternative identifier when a GTIN doesn't apply (e.g., a manufacturer-supplied replacement part with no retail barcode).

Identifiers the platform assigns to you

  • ASIN — Amazon's own identifier, generated automatically when a new product is added to Amazon's catalog (from your UPC/GTIN plus your listing data). You don't create an ASIN directly — Amazon does, based on your submission.
  • Walmart, eBay, and Etsy each generate their own internal item ID similarly upon listing creation.
  • These platform-assigned identifiers are not portable: an ASIN only means something inside Amazon's catalog, a Walmart Item ID only inside Walmart's. You can't use one marketplace's assigned ID to look up or create a listing on another.

The identifier that's entirely yours

  • SKU (Stock Keeping Unit) — your own internal identifier, used across every channel you sell on for inventory tracking. Unlike UPC/ASIN, there's no external registry — you define your own SKU format. A consistent SKU scheme (e.g., encoding product line, variant, and size) makes multichannel inventory reconciliation far easier as you scale.
  • A workable SKU convention is short, human-readable, and encodes the dimensions you actually need to sort/filter by later — for example CFPRS-BLU-34OZ for a blue, 34-ounce French press in the "CFPRS" product line, rather than an opaque sequential number that tells you nothing at a glance.
  • The same SKU should map to the same physical product across every marketplace you list on — this is what lets a multichannel inventory or feed management system (see Feed Management Fundamentals) reconcile stock levels across channels without manual cross-referencing.

When you might not need a new UPC

If you're adding your offer to a product that already exists in the marketplace's catalog (see How to Determine Whether a Product Already Exists), you don't need your own identifier — you're matching to the existing ASIN/item ID. Some marketplaces also offer a brand-exemption path (typically requiring Brand Registry enrollment) that waives the UPC requirement for registered-trademark brands.

How to obtain identifiers correctly

Register directly with GS1 (or a GS1-authorized member organization in your country) to license a company prefix, which then generates a block of GTINs that are officially associated with your business in GS1's database. This matters increasingly: several marketplaces now cross-check a submitted GTIN against the GS1 registry's recorded brand owner, and a mismatch (a barcode number that GS1 records as belonging to a different company than the one submitting it) is a common, hard-to-diagnose cause of a listing being rejected or suspended after the fact.

Buying a single "already generated" UPC from a third-party reseller is cheaper up front but carries real risk: if that number isn't tied to your business in GS1's own records, a marketplace verification pass can flag it later, sometimes well after the listing has been live and selling.

Common mistakes

  • Buying a cheap, unregistered UPC instead of licensing a proper GS1 prefix, only to have it flagged during a later verification pass.
  • Assuming a platform-assigned ID (ASIN, Item ID) is interchangeable with a UPC/GTIN — they are not, and you cannot submit an ASIN as if it were a UPC on another platform.
  • Reusing the same UPC for two genuinely different products (e.g., after a packaging redesign that changes the actual contents) — a barcode is supposed to identify one specific product configuration.
  • Not keeping a record of which identifier maps to which SKU across every marketplace, which turns a routine feed update into a research project once your catalog grows past a handful of items.

FAQs

  • Do I need a separate UPC/GTIN for every size or color variation? Generally yes — each distinct sellable configuration (each child listing in a parent-child variation structure) typically needs its own identifier, since it's a genuinely distinct product from a fulfillment and catalog-matching standpoint.
  • Can I reuse a UPC after discontinuing a product? Best practice is no. GS1's own guidance and most marketplaces' expectations are that a GTIN should not be reassigned to a different product, even after the original is discontinued, to avoid corrupting historical sales/review data tied to that identifier.
  • What if my category doesn't require a GTIN at all? A small number of categories (certain handmade, custom, or single-unit categories) don't require one — check the specific category's requirements before assuming you need to buy one.