I have no idea how they get the keys (probably OEM) and whether this is actually kosher (and I don't wanna know) but I did this with Win 10 some years ago as well and it has worked out thus far.
Just pay with e.g. PP and they'll send you the key via e-mail.
Grey market key resellers, apparently using volume licensing. It's a roll of the dice whether they'll stay valid, possibly because the same key is resold repeatedly, or Microsoft blacklists them.
I have no idea how they get the keys (probably OEM) and whether this is actually kosher (and I don't wanna know) but I did this with Win 10 some years ago as well and it has worked out thus far.
Just pay with e.g. PP and they'll send you the key via e-mail.