It may happen that the GTIN you entered is already tied to an existing listing (ASIN) as shown below:

Another example: 

If this is the case it means that amazon has allowed another seller (likely a scammer trying to avoid paying for real barcodes) to list items using your GTIN. It should go without saying that this is illegal and the offending party can be sued for damages. 

While there are roughly 1 Trillion GTIN's globally created by GS1, it would seem like it's hard for someone to guess your unique barcode number, however the opposite is true. Most scammers that do this sort of thing are not trying to guess YOUR code, they're just randomly making up #'s (ten's of thousands of #'s) and seeing if they have not been used yet on amazon. So through trial and error they eventually find unused GTIN's and exploit amazon's automated platform that can't detect fake items. In the past this type of rampant fraud was a sizable issue, it is less so these days. 

However, the one thing that amazon does not do is remove the illegally used listing & barcode from their permanent "used" database. This single fact is the crux of their poorly thought out system and the cause of countless issues since. If they can suspend a seller for scamming, why would they not remove what that scammer created/used/stole? Instead they keep all of these scam listings on the back-end, called "tomb-stoned listings" which prevents the eventual legal owner from being able to use their GTIN's in the future.

Amazon is well aware that this can happen with the way their POS system is setup. Their seller support is legally obligated to remove illegal listings and will suspend the seller that is illegally using them, if they haven’t been suspended already.

Important Note: Illegal usage on amazon does not mean it's used anywhere else. Store's POS systems do not connect to the internet, other stores or websites, so if a scammer has used your code on amazon it would not negatively affect your use anywhere else globally. Amazon is one of the only online venues that have an open platform with as few human-in-the-loop safeguards in place to keep this sort of thing from happening, which is why so many scammers target amazon's popularity.

There are a few solutions to illegal usage:

1: Easy, cheap & instant solution: (What most sellers do) Since barcodes cost cents, simply use a different barcode #/GTIN and continue on with the listing, thus side-stepping amazon's defrauded catalog.

2: Fairly Difficult, time consuming but free solution: File a formal complaint to seller support, more on this topic (HERE) and wait for them to possibly respond. 

3: Most difficult, most expensive & most time consuming solution: Have your legal counsel send amazon's legal department a formal letter stating the facts, requesting to restore your GTIN to working order, providing the documentation from your purchase from Bar Codes Talk. 

 *It's important to note that the seller has done the illegal act, not amazon. This allows amazon to claim they are not obligated to do anything but may be able to help if petitioned.