In almost all cases, the cheap license is illegal, whether volume license, MSDN key, or TechNet key. Except Microsoft does not bother to chase retail customers who knowingly or unknowingly buy and use such keys. They are not meant to be sold to end users like retail license or OEM license.
If I am not mistaken, volume license are only meant to be used for large companies, institutions, and organizations that bought many licenses for use on their PCs internally. If your computer does not belong to the licensing organization, it is illegal.
For Sim Lim Square kind of IT mall, at least they can claim to be bundling the OEM license with a new DIY PC, which makes selling OEM license legal.
Usually Microsoft do not bother.
They can check and verify when you do the Updates.
The other point is that thete is a limit of licence.
Once the lot of licence issue are used up, the next activation will failed.
My SLS technician gave me the installer but told me either not to pass to anyone else &or Update the software.
I did bought a Win 7 OS online without disc n cert.
Recently one failed as illegal by Microsoft when i power on the laptop.
The Online sent me another key, and told me not to set auto Windows Updates.
Although the new key work but somehow i foul up.
In the end i used the original 1st time installation, where i did a clone.
It did work when i reused the original, as means of good backup.
But do note that once you have the key, quickly activate it.
This is advice by that Online seller.