Can I sell on Amazon FBA listings which are no longer being used by companies which left the marketplace?

This is not a simple yes or no answer, depending on a few things:
– If you are selling the same item with the same UPC, yes you can sell on the existing listing. But that cannot be a brand gated listing.
A brand gated listing means the official manufacturer has presented Amazon with their Trademark and requested to lock that listing. An easy way to check this is to download the free Amazon seller app and open the listing on your app (It will tell you)

– If the product is used or new, that also makes a difference, as certain brands do not allow sellers to sell brand new items of their brand, but allow the sale of used

– If the product you want to sell is not the same brand, or doesn’t have the same UPC, you are not allowed to sell on that listing!
A lot of sellers started doing that a few years ago, trying to use existing reviews for a new product, and those accounts have been suspended.
Hope this helps, and if it does, please upvote 🙂

Selling on Amazon FBA listings which are no longer being used