I've bought a couple keys now from
GoDeal24 and they've all worked fine. Around $10 a key give or take for an OEM key. That link is actually to their weekly "sponsored" fake-article ad on Wccftech just because it has whatever the current coupon code is in it. Read down to where it explains how to use Paypal. Basically, it initially gives you one payment option ("CWallet"), you don't have a CWallet account and then it gives other payment options. Then you pick Paypal because you're not the sort of lunatic to give your credit card information to a website that sells $8 Windows keys.
I've bought keys for as low as $1 from AliExpress and they worked but you had to go through some rigamarole to activate them. I feel more comfortable that a $10 is probably a resold OEM key or key from cheaper market versus a $1 key or $5 Kinguin special which was probably cracked and runs more of a deactivation risk. That said, I've run the spectrum and not had a problem yet with any of them. The only reason why I personally care is because some of the keys were used on systems for family or one I sold and I don't want anyone coming back to me with "Windows says it doesn't work..."
If a Windows key ever was revoked, all it does is throw a nag in the screen corner to activate it and locks you out of things like changing the system wallpaper. You wouldn't lose your computer in any meaningful sense. So I figure that, even if a key WAS revoked, you can buy a whole lot of $10 keys and find one that works before you hit the cost of Win10 off the shelf at Micro Center. But, again, hasn't happened to me yet. For reference, Microsoft sells "retail" Win10 Pro on their Friends & Family employee store for $20 vs the $150 shelf retail price and I'm sure that twenty bucks is still designed to turn a profit.