From my experience reading manufacturers' warnings[1] about this, they never use the phrase "gray market" because a lot of consumers don't understand what that terminology means. It's more that the dealers/retailers and some savvy consumers use the phrase "grey market".
There is a category of _new_ (not used) products that have the following attributes:
- sold by unauthorized dealer : i.e. no contractual relationship with the original manufacturer to buy from them and then resell the item
- sold without a manufacturer's warranty : any warranty must be honored by the unauthorized reseller or some other 3rd party or comes with no warranty at all
So instead of the cumbersome long-winded hyphenated term of "unauthorized-retailer-and-no-manufacturer-warranty market", the industry just shortens it to "gray market". It's a useful label of what the product is. You're right, it's not unlawful or unethical to sell unauthorized/unwarranted merchandise as long you inform the customers.
E.g. Most of the luxury watches sold on amazon.com (Cartier, Rolex, etc) are "gray market" because they don't come with manufacturer's warranty. If you buy a Cartier watch from Amazon[2] to save money and it's broken and needs repair, you can't take it into a Cartier retailer at the mall to have them fix it. There's nothing wrong with it as long as customers understand the tradeoffs of lower price vs service convenience. Even though the watches are genuine instead of counterfeit, they don't come with manufacturer's documentation to get factory-authorized service.
[1] one example of a manufacturer warning about "gray market" without ever using that terminology: https://usa.yamaha.com/support/unauthorized_sellers/index.ht...
[2] https://www.amazon.com/s?k=cartier+ladies+watch&s=price-desc...