timba91 :
You can buy cheap keys from trusted European online shops no problem. There are some laws here that make it legal(reselling software cannot be illegal). I paid 7,50E for my retail pro key and it's worked for over a year now.
It's not the 'reselling' argument that is overly cause for concern - although technically against the EULA, regions have ruled it cannot be legally enforced. That legal argument though, IIRC, is for software you legally acquired for personal use. So a prebuilt desktop that shipped with W10 Home (for example).
The issue with 'grey' keys, is they have potentially been bought with stolen credit cards.... Or a volume license that has a limited number of activations, but continues to be resold.
Both of those examples (and many others, I'm sure) can fully, and legally be deactivated by MS.
Your sample size of 1, doesn't really validate anything - take a browse around this forum for specific sites that are known for selling keys. Some interesting reading.
Ultimately, they're buy at your own risk.