Whatever link I used when I did mine, I ran the seller through several searches and also asked how it could be that price, etc. They are large commercial licenses. That's the bottom line. At the end of the day, the way I know it's legal is that I also ran it through Microsoft, which validated my purchase of a licensed copy. Note I am speaking in general because I am not sure which seller you are referencing as compared to the one I used, but a bit of research on the seller etc. should do it.

I'll add that I thought to do this after seeing how Lifehacker is always advertising "you can buy a licensed copy of _____ for only $30 this week only" and takes you to their affiliate. I bought an MS Office license there once also, BUT it was almost out of date within 3 months. But that made me wonder if there were other wholesalers. I ended up on a bunny trail. I definitely found some unlicensed versions as well, but as soon as I started running those through searches and reviews it became clear who was legit. To me, it's worth an hour or two of front end research to own the copy.

Groupon is good about refunds and/or credits if a seller scams you too. I do plan to buy a new copy this weekend for my new computer, so if I remember, once I look again, I will report back!