There are also plenty of authorized resellers on eBay offloading unused corporate Enterprise LTSC keys.
There are no "authorized" resellers. There are ONLY Microsoft authorized "partners" and those are the ones selling legitimate licenses, not second hand, stolen, batch obtained or corporate package licenses. Believe me, as even the top brass at Tom's hardware have learned, I've had discussions with some folks at Microsoft at a fairly high level and I ASSURE you that aside from the authorized partners and Microsoft itself, there are ZERO authorized "resellers". None. Nada. Zip.
The fact that Microsoft isn't actively hunting them down and forcing them out with legal action, because it's not cost effective, does not in any way mean that that makes them legit AND the backstory to this is that they don't have to do that anyhow because they accumulate product keys that are listed as being part of stolen shipments, corporate licenses that have "disappeared", commonly reused licenses, etc., and periodically simply blacklist them in batches so that your 7 dollar license you bought and used for six months or a year and was seemingly legitimately activated, suddenly becomes not-activated, and there is not one damn thing you can do about it.
I've even seen this happen to a user after three years of using the key on the same system. Nobody to blame but himself, which is exactly what he was told.
Now, obviously not all licenses will get caught up in this way, but a lot of them do, so when you buy that kind of crap not only are you telling the people who are stealing the product by the pallet load off the backs of trucks, out of boxcars, doing it digitally, or whatever, that it's ok to do that and keep on doing it (Which obviously results in companies charging more for the product to offset these losses which directly harms THE REST OF US who actually pay for products we appreciate and want to see continue to be supported, but you are also playing a game of craps and rolling the dice on the belief that YOU won't be one of the ones that gets their license blacklisted. Trust me, we've seen it HUNDREDS of times, and those are only the ones we SEE. Certainly it is far more widespread than we ever see or hear about. So if you want to take that chance and support the organized crime syndicates that perpetrate this kind of crap, that's your choice, but recommending that others do it is a good way to get a stigma attached to your name.
I assure you, there were those from on high at Tom's that tried to argue this crap with me on a few occasions, and while I can't win the argument against them talking it up (cheap keys that is) in their editorial side and articles, believe me, after I presented the evidence directly from MS to them, it was the last I ever heard regarding the legitimacy of those kinds of "gray" keys.