Cheap licenses will mostly activate and if they don't, usually if you contact the seller they will give you another one. They activate, because they are genuine licenses. But despite that, they are being sold illegally. You can't ever legitimately buy licenses for Microsoft products cheaply, aside from student discounts. One notable exception to this, is that in the EU, a court ruling made it legal to resell OEM licenses. But these cheap used product keys can only legally be used in the EU.
At least in the case of Windows 11, Microsoft lets you upgrade from Windows 10 for free.