i've not heard of anyone being charged or sued for buying those keys. however, you are concerned for a reason and it is very valid.
there is no way something that costs $100+ can be legitimately sold for $20 or less!! it's just not possible. you do not see this anywhere else in the economy. not like you can buy a new car for $40,000 from the dealership or $1000 from some back alley "seller" and not KNOW it was totally either a scam or stolen goods.
that's exactly what those sellers are. either scams or selling stolen goods. why would you want to support such a thing in any way? MS sells the OS for a certain cost even to builders and it costs what it costs. anything WELL BELOW those costs is not legal and should not be supported...