In my head all I hear is "Yeah, MSDN or VLSC codes that are soon to be deactivated."

Anyone want to lay odds that this EE add is for legit product?

I don't even pretend to know the infinite twists and turns of MSFT licensing but if I had to wager I'd say that the keys might be valid but the sales channel, seller, sourcing, and pricing sure as hell doesn't look legit to me.

I'm surprised it was allowed in the EE unless the seller was able to show some docs or proof that this was an authorized sales/licensing channel and pricing.  Which again, hardly seems likely.

As they say, not my monkeys, not my circus.

I actually have a challenge coin that says this. :-)

I need a stack of those so I can flip one to everyone I say that to.

Pretty good chance of them just being volume/enterprise activation keys. So they will work as long as the person/company who actually owns them(the main license they are based off of) keeps their license active. The activation key would become invalid after that, at least that's how it use to work.

If he's not providing the COAs with the key printed on them, they're not legit.  He says they're OEM, and the OEM HAS to have the sticker, because it's married to the box.

They're not exactly cheap, so I hope you don't say it too often, or you really like the people you say it too. 
Schlock Challenge Coins

I hate to say it, but I got 'a few' via kickstarter when they first came out. 

On one side, I really can't stir up much sympathy for MS losing out on a bit of hobbyist revenue.
On the other side, I am not a fan of buyers being taken advantage of.
On yet another side, $18 is not a high price for a valuable lesson on 'if the deal is too good to be true..."

Overall, I care just enough to grab some popcorn & watch. 

So instead of reaching out to the seller and asking for information you decide to post up here and have everyone speculate.  Could also be construed as ad stomping so be careful of that.

System Builder licenses are still a thing.
https://www.amazon.com/dp/B00ZSI7Y3U?tag=vglnk-c102-20

What makes me go  is he says he does not provide a COA (cert of authenticity) which is required for OEM licenses as I have bought them before.

He probably got the keys from scdkey or a similar site. I am sure they will activate windows but who knows how legit they are.

The keys are probably not suppose to be sold to the US market. I am sure Microsoft does not sell windows for the same price in every country. The keys are probably sourced from Asia.

I have seen them as low as $13.xx.

scdkey windows 10 link

Everything here is supposed to be legal and legit. I am sure Microsoft is not happy about it because it's probably gray market, etc.

Microsoft legit keys selling on Reddit

That is indeed a grey market, and is semi-legit (German high court ruled that MS *has* to allow end-users to resell licenses just like any other capital asset, but there is no legal precedent under US Law, yet) but the exact issue with the OPs link in question is that those are OEM keys.  OEM keys can be purchased, but MUST come with a Certificate of Authenticity sticker that is attached to the computer the OS is installed on, because once installed it is permanently married to the box and is non-transferable.  So, to buy an OEM key without a COA is software piracy.

Be careful of discussing a perfectly reasonable question about a subject that this forum is exactly for?  What's being done is a VERY common thing around the fringes of enterprise IT and regardless of whether it's high-end networking gear on eBay or volume license keys, it's nearly always against the licensing and use terms of the manufacturer and it leads to people winding up with products that they can't get support for and often lose access to because of various license renewal and rollover schemes.

More to the subject, I have personally seen 3 people walked out of the building (2 initially charged with felonies) over "extra license keys" and the resulting "true-up" audits that result from it really suck.  Someone always looks at the licensing numbers and thinks "hmm, we have 19,877 of 20,000 2016 Server Datacenter and 14,908 of 15,000 2016 SQL Server Enterprise licenses left before the EA rolls in 3 months.  Nobody cares.  They aren't being used and the were already paid for..."

I'm not presuming this is the case here but I've seen that very thing happen.  The "benefit of a doubt" isn't my first reaction in this particular arena.