The first sale doctrine protects your right to sell items that you've purchased... even if you want to price it at a loss.
Amazon, for example, sold ebooks at a loss for years. Obviously you wouldn't say Amazon's ebooks were counterfeit or stolen.
And Amazon does not have sole ownership of that tactic. Others have an equal right to sell items at a loss, even against the manufacturers wishes.
Other examples of when this occurs: going out of business sales; end of season liquidations; etc. and various reasons to liquidate inventory: bad buys; changing product categories; etc.
Here is another example: Crestron, they specifically do not permit sales on Amazon, ebay etc. The only way to acquire their hardware is to go via a Crestron authorized dealer or becoming one yourself. Yet there is crestron hardware on Amazon...
The reason being is that their hardware requires programming and unless you are familiar with how it works you will have a very bad experience.
It's possible that there are ways for the manufacturer to maintain control in case of an authorized seller's bankruptcy (e.g. not actually selling it to them until it's transferred to customer should work), but if they actually transfer it to them they only can demand contractual damages from them, not stop a buyer from doing anything with the product.
I know of people manually removing serial numbers so the items can't be traced back to them. I know of people setting up multiple companies, selling from one to another and providing documentation that it wasn't "them" when caught. I've seen stories of people buying undelivered shipments from carriers and selling products found.
Point is, you can't assume everything is counterfeit just because the manufacturer doesn't sell to amazon sellers.
I've been threatened in the past by a manufacturer for selling their items: in that case, I had bought some from Amazon directly when the price was lower, and some from another authorized dealer. There are a lot of manufacturers that think their rights go a lot further than they actually do.
They may not allow it, but they can almost never stop it without making a deal with Amazon.
I wonder if this is legal or possible.
I grasp that there are special cases, i.e. complex gear ...
But if you own a piece of gear outright - I don't think there is anything anyone can do to stop you from selling it to someone else.
Unless someone cares to comment ...
There are many items sold on Amazon that the manufacturer doesn't sell direct nor sell to sellers to sell direct, but somehow people still manage to get stock.
Amazon isn't going to take down a listing unless the manufacturer can prove that it's infringing. They will block specific sellers if you can show they're selling fakes, and it's not that hard to do (place a test order, then report).