The main legal issue is with the seller as they may be selling illegal keys, not so much with the buyers.
However, Toms' does not condone nor endorse any illegal or potentially illegal activity, nor users taking part in it.
If you are concerned and also want to potentially save yourself money and trouble in the long run, buy a legitimate copy from Microsoft (or just get a legitimate Windows installation and don't activate it).
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.
use it without a key if you have to. you lose no functions and only have to use the default desktop. not a big deal for free really.