It says:
Windows 10 Professional 32bit/64bit Global License Product OEM Key
And shows a price of $37.95 and you have to download it.
Is this for real?
I will put the link here if you need it, but just Google and you should find it.
He is a place that does not sell it, but talks about it:
http://www.ghacks.net/2016/09/03/getting-a-windows-product-key-on-the-cheap/
When I go there I get a pop-up on the right with a different price.
What is the CATCH?

EDIT: The link I gave is a seller. It was not obvious at first.  They are likely NFR keys stolen from a manufacturer or places like MSDN.Yeah, they aren't going to be keys that are allowed to be sold.  They will more than likely work but the keys will have been acquired from something like MSDN or OEM keys ripped off of scrapped machines.  By buying these, you aren't funding Microsoft, you're just giving money to the seller.  The copy of Windows will most likely activate and work but it's no better than running a pirated copy.

The key thing to remember is that for Windows licences, Microsoft sets the pricing.  If someone is selling them for huge discounts then the seller is either being very generous and selling them at a huge loss or are selling dodgy keys.  You can also often spot in the fine print of the listings nonsense along the lines of "This is a genuine key recovered from a scap PC.  As per eBay's terms and conditions, you may collect the scrap PC at your expense" - Even with that nonsense disclaimer talking about a PC that almost certainly doesn't exist, the key is still not permitted to install on any other machine.You said:
Quote

the key is still not permitted to install on any other machine.

by 7redeye Aug 08, 2016
It depends
The version of Windows 10 Pro was a pre-NOVEMBER 15 version. Original installation was OK, but it HUNG up when it got to the November 15th upgrade, which was equivalent to a complete reinstall. It took me 2 months of working with MS, trying stuff suggested by the internet, etc. I finally downloaded the Nov. 15 upgrade to thumb drive and installed it from the USB. Since then, everything works great. I think that Win 10 Pro is far superior to Win 7. I suggest if you buy a Win 10 DVD, get one made after Nov 15 2015. Any download from MS (if you just purchase the key code) now should be OK.