I guess if things get ugly they have some (terrible) plausible deniability? “What do you mean my $10 Windows Enterprise key is stolen? I thought that was the going rate. Linux is free”

Who is violating the terms? The seller or the buyer?

If it’s the seller, would Microsoft go after the buyer?

Both. Microsoft would probably never go after the buyer, but I just wondered the justification people buying from the grey market are - legally speaking in many places, you're just paying money to pirate it - it's not a valid license.

It's even more pointless now that I've just given up on Windows and use Linux.