I didn't even know that microsoft is not selling Keys for home use, well, now i know. So if someone want's to sell me some microsoft software, what do i have to check to make sure it's ''legal'' and ok?

With the exception of Product Key Cards distributed with Certificates of Authenticity (COA’s), Microsoft does not distribute products keys as standalone products. If you see a listing on an auction site, online classified ad, or other online page advertising product keys, it’s a good indication that these keys are likely stolen or counterfeit.