Product Identifiers: What GTIN, UPC, EAN, ISBN, ASIN, SKU and MPN Mean, and Which Ones Amazon, eBay, Walmart and Shopify Ask For

A product identifier is a code that lets a marketplace recognise exactly what you are selling, a fingerprint for the product. They fall into two families: global codes issued by GS1 (UPC, EAN and the GTIN standard that covers them) that mean the same thing everywhere, and marketplace or seller-assigned codes (ASIN, SKU, MPN) that only mean something on one platform or inside your own system. This guide covers what each one is, where to get it, and which marketplaces require which.

The identifiers, one by one

Amazon barcode requirements covers FNSKU and the physical labelling of FBA units in full; this post stays on what the identifiers are and which each marketplace asks for.

Where to get identifiers, and why cheap UPCs get listings blocked

UPCs and EANs come from GS1, the official issuing body, through a company prefix licence or a single-GTIN purchase. Amazon and other major marketplaces verify submitted codes against the GS1 database, and a code that does not match GS1's records is flagged as invalid whether or not it scans.

The resold-UPC problem: cheap codes sold outside GS1's own licensing are often codes originally issued to a different company whose GS1 membership lapsed. GS1's database still shows the original company as the licensee, not you, so Amazon's check fails. This is the most common cause of a rejected new listing that otherwise looks correct. Buy from GS1 directly, and if a code came with a product from a supplier, check whose name it is registered under before you list.

Brand owners without a GS1 code (handmade sellers, some private-label cases, parts sold under a manufacturer number) can apply for a GTIN exemption instead of buying a UPC; the Amazon barcode requirements post covers the exemption process.

Amazon product identifiers: ASIN, UPC, EAN, FNSKU and the item number

At listing, Amazon asks for a UPC, EAN or other GTIN for any new product not already in its catalogue. Amazon first searches the code for a match: if the product exists, your offer is attached to the existing detail page; if not, a new page is created and Amazon assigns it an ASIN automatically. The ASIN is what Amazon uses internally to track the product, and it is specific to each Amazon marketplace, so the same item can carry one ASIN on Amazon.com and another on Amazon.co.uk. Accurate identifiers also feed Amazon SEO, because a listing matched to the right catalogue record inherits its search relevance and reviews.

The "item number" a shopper sees on an Amazon product page is the ASIN under a friendlier label. If you are a buyer looking for the item number on your own order rather than a seller listing a product, this guide is about the seller side of the identifier system, not order tracking.

The physical barcode that goes on the unit for FBA is a separate question from the UPC used to create the listing; that is the G11b post above.

eBay: MPN, UPC and the item number

eBay does not strictly require a product identifier in every category, but including one (a UPC, GTIN or, when no UPC exists, an MPN) improves how a listing surfaces in search and how well it matches eBay's product catalogue; in categories such as electronics and branded goods it can affect ranking and conversion directly. The MPN field is for products without a UPC, common for parts, accessories and some private-label goods, and lets eBay still match the listing to a catalogue record.

eBay's own "item number" is a listing number eBay assigns; it identifies the listing, not the product. The how to sell on eBay post covers listing setup from the start.

Walmart, Etsy, Shopify and TikTok Shop

Walmart requires a GTIN for most new-item listings, more strictly than eBay, because its catalogue matching depends on it; the how to sell on Walmart post covers the onboarding side.

Etsy does not require product identifiers at all, since most Etsy listings are handmade or custom items with no UPC to begin with. Shopify has an optional barcode field, useful mainly if you sync inventory to Google Shopping or to a marketplace channel that does require one, and it keeps syncing accurate across channels. TikTok Shop treats GTIN as optional for most categories but recommends it for catalogue matching.

SKUs: designing your own

A SKU is entirely your own system, so the only real requirement is consistency: one SKU per specific product variant, used identically on every channel you sell on. A structure that encodes something useful (category, launch date, a size or colour code) makes the SKU readable at a glance rather than an arbitrary string, and it is the SKU that keeps stock counts right when the same variant sells on Amazon, eBay and your own store.

Two problems identifiers cause when they go wrong: duplicate listings, when the same product is listed under two sets of identifiers (consolidate them under the correct code before a marketplace does it for you), and a failed ASIN match, when your details do not line up with the catalogue record (check the code and the product details, then contact Seller Support).

Why identifiers matter after the sale

A support ticket that carries the SKU and, for Amazon, the ASIN lets an agent identify the exact product and find the order quickly, whichever channel the message came from, and a buyer who received exactly what the listing described is a buyer who does not need to return it. Getting identifiers right at listing is not only a compliance step; it is what makes a support team fast later. See Amazon helpdesk software for how eDesk keeps the order and product on every Amazon ticket.

FAQ

What is a product identifier?

A code that lets a marketplace recognise a specific product: either a global code (UPC, EAN) issued by GS1, or a marketplace-specific code such as Amazon's ASIN, or your own SKU.

What is the difference between UPC and EAN?

Both are GS1-issued codes serving the same purpose. UPC is 12 digits and used mainly in the US and Canada; EAN is 13 digits and used in Europe and most other markets.

What is an ASIN?

Amazon's own 10-character alphanumeric identifier, assigned automatically to every product in its catalogue. It is specific to Amazon and can differ for the same product across Amazon marketplaces.

Do I need a UPC to sell on Amazon?

For most new products, yes, unless you qualify for a GTIN exemption (handmade goods, some private-label cases, parts sold under a manufacturer number).

What is an MPN on eBay?

A manufacturer part number, used on eBay when a product has no UPC; common for parts, accessories and some private-label items.

What is the item number on Amazon?

It is the ASIN, shown under a shopper-friendly label on the product page.

Once your listings are set up correctly, keep the SKU and ASIN in view where support can use them

See Amazon helpdesk software for how eDesk connects to Amazon.