ASINs, Barcodes and Product Identifiers

Four codes come up in the first week of selling and they do different jobs. Getting them confused is how a seller ends up with a listing on the wrong product, or no listing at all.

The short version: a barcode identifies the product in the world, an ASIN identifies the page on Amazon, and an SKU identifies the stock in your own account.

ASIN. Amazon's own identifier for a detail page. Ten characters, assigned by Amazon, not by you. Every offer on that page shares it, which is the whole point: one product, one page, however many sellers.

GTIN. The family name for the barcode standards. A UPC is the twelve digit version common in North America, an EAN is the thirteen digit version common in Europe, and an ISBN is the one books use. They are all GTINs.

SKU. Yours. A code you invent to identify your own stock, and it never leaves your account. Two sellers on the same ASIN have different SKUs and neither knows the other's.

FNSKU. The label FBA puts on a unit so Amazon knows whose it is in the warehouse. Since 31 March 2026 a reseller has to put one on every unit, and it matters when a return comes back.

Every detail page shows its ASIN in the product information section, and it is also in the page's address, the ten characters after /dp/. For most printed books with a ten digit ISBN, the ASIN is that ISBN, which is why booksellers rarely meet a separate number.

Before creating anything, search the catalogue from the add a product page in Seller Central, first by the barcode on the product, then by name and brand. A barcode that finds a page settles the question: the product exists, and the offer goes on that page. A name search that finds a close match needs reading rather than trusting, for the same pack size, the same colour and the same model. An offer placed on a page that describes something slightly different is a not as described complaint waiting to happen, and the complaint lands on whichever seller shipped the unit.

The search answers two more questions. Whether the brand restricts who may sell it, which shows as an approval requirement before the offer can be created and is covered in brand approval and ungating. And who controls the page, which decides whether a correction to it is yours to make, as how the Amazon catalogue works explains.

When the product does not already exist on Amazon. Creating a new detail page means proving the product is a real product, and a GTIN is how that is done.

When the product does exist, you do not need one. You add your offer to the existing ASIN, which is the correct behaviour and the one new sellers most often get wrong by creating a duplicate page for something already listed.

Buy barcodes from GS1, the body that issues them, rather than from a reseller. Amazon checks the number against the GS1 database, and a cheap barcode registered to somebody else's company name is refused, sometimes months after it was used.

A barcode identifies one sellable item, so every distinct item needs its own. Each size and each colour in a variation family is a separate product with a separate GTIN, even though shoppers see them on one page. A multipack is a different product from the single unit: a pack of three needs a barcode of its own, not the single unit's barcode used three times.

Bundles follow the same rule. Amazon's product bundling policy expects a bundle of different items to carry a barcode of its own, and the barcodes of the items inside it cannot be used to list it. Changing what is in the bundle makes it a different product, which means a new barcode and a new listing rather than an edit to the old one.

This is where small brands most often run short of barcodes, because a range of five products in three sizes with a multipack option is not five codes but thirty or more. Count the sellable items before buying a block of barcodes from GS1, and keep a register of which code was assigned to which item, because a code assigned twice is the same problem as a borrowed one.

Some products legitimately have no barcode: handmade goods, private label lines that are genuinely new, parts and accessories sold by a brand that does not barcode them.

Amazon grants an exemption for those, per brand and per category, on evidence: usually images of the product and its packaging showing the brand, with no barcode present. It is a real route and it is slower than buying a barcode, so it is worth starting early if you need one.

Amazon's help page for the request sits behind the Seller Central sign in, and the route to the form has moved more than once, so here is what the request asks for rather than where the button is. It names the brand, or says the product is unbranded, and the category it will be listed in. It lists the products. And it attaches photographs of the product and its packaging, from several sides, showing the brand name and no barcode.

An exemption is granted for a brand in a category, not for a single product, and each Amazon store grants its own, so a brand selling in two categories, or in the US and the UK, applies for each. Where the category is itself restricted, approval to sell in it is a separate step.

Two details decide whether the listing then works. The brand name on the listing should match the one approved exactly, because the exemption is tied to it. And the photographs should be of the real, finished packaging: a mock-up or a render invites the question of whether the product exists yet.

An exemption is the right answer for handmade goods and genuinely unbranded lines, as the guide to getting a GTIN says too. For a brand that expects to sell through retailers, it postpones the barcode rather than saving it, because most retail buyers ask for GTINs.

A duplicate page for an existing product. Splits the reviews, splits the sales history, and eventually gets merged by Amazon or reported by the brand. Search the catalogue before creating anything.

A barcode registered to another company. The listing works until it does not, and then the listing is removed and the stock is stranded.

Reusing an SKU. Your own code, your own problem, and it is a bad one: sales history, stock and returns all attach to the SKU, and reusing one for a different product makes the account's own records untrue.

Assuming an ASIN belongs to you. It belongs to the product. Even a private label ASIN you created can carry other sellers' offers, which is what reselling allows.

Stock sent to Amazon's warehouses has to be identified on every unit, and since 31 March 2026 who sends it decides how. A brand owner with the Brand Representative role in Brand Registry can send products that carry a manufacturer barcode, such as a UPC, without Amazon's barcode stickers. A reseller who is not a Brand Representative has to label every unit with Amazon's own barcode, the FNSKU, even where a manufacturer barcode exists, and a product with no manufacturer barcode needs the FNSKU whoever sends it.

The change ended what sellers call commingling. Until then Amazon could treat identical units from different sellers as interchangeable and ship the nearest one, so another seller's counterfeit or damaged unit could reach your customer and the complaint land on your account, which was the reason most brands labelled. Amazon says it has ended the practice across its supply chain, so the label now follows who sends the stock rather than the risk.

Whichever is chosen, the label has to be scannable and must cover any other barcode on the unit, because a unit with two readable barcodes can be received against the wrong one. Most receiving problems at a fulfilment centre come back to a label: printed too small, placed on a curve, or covering nothing.

GS1 runs a public lookup, Verified by GS1, that answers the question a marketplace asks of a barcode: which company assigned this number. GS1 says anyone can check a GTIN there, and a search returns at least the company, organisation or person that assigned it, with the product's details where the owner has entered them.

Three moments call for it. When a factory offers to supply barcodes with the goods: the numbers will come back assigned to the factory, not the brand, and a brand building its own catalogue is better off with numbers licensed in its own name and matched to the brand on its listings. When barcodes were bought cheaply in the past: the lookup shows whose prefix they came from, which is the record Amazon reads. And when buying another brand's product to resell: the number on the box should come back to that brand, and one that comes back to somebody else is a question for the supplier before the stock is bought.

A number GS1 cannot find is a stronger warning still, because it may not be a GS1 number at all. And a brand checking its own codes should see its own company name, spelled as it is on its listings. Where it does not, the record at GS1 is the thing to correct, before a marketplace reads it.

The same GTIN travels. Walmart and eBay both accept the standard barcodes, and both use them to match your offer to their own catalogue, so a product properly identified once is easier to list everywhere afterwards.

What does not travel is the ASIN. It is Amazon's, it means nothing anywhere else, and a catalogue keyed to ASINs rather than to GTINs is a catalogue that is hard to take to a second channel.

Every product ends up with a handful of codes, and they belong in one place. One row per sellable item: its GTIN, its SKU, its ASIN on each Amazon marketplace, its item IDs on Walmart and eBay, its FNSKU if it is labelled for FBA, and what it is, in words. It sounds like administration, and it is the document every later job asks for first.

It is what makes a second marketplace a matter of weeks rather than months, because the listings can be matched by barcode from the register instead of rediscovered. It is also what an order management system is set up from, as order management across marketplaces explains, and what an auditor, a buyer of the business or a new agency will want to see.

Keep it current as products change. A discontinued item keeps its row, marked as discontinued, rather than being deleted, so its codes are never reused by accident.

Getting identifiers right at the start is unglamorous and it is the difference between a catalogue that can be moved to another marketplace and one that cannot, which is why it is part of setting up an Amazon account properly.

It is also the first thing we check when a listing on Walmart or eBay will not match: nine times out of ten the barcode on the carton is not the barcode in the feed.

If your catalogue grew by hand and nobody is sure which codes are right, an audit is the cheap version of finding out later.