What many sellers on Amazon forget is the legal concept known as the First-Sale Doctrine. Simply put, you get the right of first sale but after that, resellers are legally allowed to sell your product.

So, if the product is legit (which you stated it is) then they originally purchased it from you (probably on sale, like you mentioned) and they can now legally sell that item themselves.

Sellers like to complain and state that Amazon isn't "protecting their brand" but Amazon is bound by this legal doctrine and can't do anything about it. You, as the brand owner, are responsible for control of your supply chain. If you sell an item at too steep of a discount, then a reseller buys your item and undercuts your price, that is your responsibility.

Here's a quick snippet on the First Sale Doctrine, but it's pretty easy to find through an internet search as well.

"The first-sale doctrine (also sometimes referred to as the "right of first sale" or the "first sale rule") is an American legal concept that limits the rights of an intellectual property owner to control resale of products embodying its intellectual property. The doctrine enables the distribution chain of copyrighted products, library lending, giving, video rentals and secondary markets for copyrighted works (for example, enabling individuals to sell their legally purchased books or CDs to others). In trademark law, this same doctrine enables reselling of trademarked products after the trademark holder puts the products on the market. In the case of patented products, the doctrine allows resale of patented products without any control from the patent holder."