Buying links from others could penalize you

I saw a thread from Digitalpoint forums, that buying links from suspected ones could be illegal and Google will penalize you and your website for buying such links that are not wanted. This is not fair in order to increase your search optimising strategy by building more backlinks to your own website or blog because it's under the Google's Terms of Service (TOS). Google so far is very strict of unwanted links from suspicious dealers that will only gain you backlinks illegally. I really don't know why, but could it be the same as Text Link Ads or Backlinks? I'm not really sure, but those two there are legitimate and different from those scammers who sell links illegally for search optimising purposes and earning extra income.