€3 keys off eBay work perfectly fine and are legal since reselling OEM software has been explicitly ruled to be legal several times in EU courts now.
The ruling was appeal and a 2016 ruling in a higher court just confirmed that you can in fact resell software licenses.
That said, sometimes the licenses can indeed get cancelled. I’ve seen a really sketchy looking photocopied label that a guy “got” that didn’t work. Depends on the channel perhaps.