Microsoft tightly controls the pricing and distribution of its operating systems.
Legally XP and 2000 can be sold, with the purchase of computer hardware (OEM copy) for $134.00 U.S.
These cheap deals are copies of the operating system, which means one day you are going to be in for a surprise when you have to re-activate the OS and can't.
Or worse, they won't even ship you a copy. Just take your money and run.