As someone who has had a few TN subscriptions...it is VERY not legit to sell them and MS is getting more creative on the ways they will determine how to shut them down.
I could generate 10-20 keys of virtually any OS / Product they had on demand and in theory they "are good forever". However, it comes with a caveat that they will only be used to dev/lab/testing and not for any production or technically personal use.
Lots of people have their keys turned off wen they see activations occuring in different areas/regions/ or just in certain patterns. Most guys don't keep the subscription active after the first year they generate the keys, so you can't really fight it either.
Tread cautiously.