If I say it, it could be false. If someone else says it, you might believe it.

I try to impress on clients that many vendors (of the larger variety) who sell Back Links should be viewed with suspicion.  Link Building is a precarious service because it can cause great harm if done incorrectly.  Here is the “someone else” (Michael Gray published at SearchEngineJournal):

…search engines consider buying links, with cash or cash in-kind exchanges (like buying someone a new iPad) is a violation of their policies. This violation may cause the link to get discounted on the low end of the penalty scale, or have the site completely removed from the index if Google thinks the violations are flagrant. I’m not going to claim that I’ve never paid for links or be naive and tell you that they don’t work. However I am going to say it’s high risk behavior. If you do it, you should be fully aware of what you are doing, and know the penalty if you get caught.

The problem is most people buy links in a stupid manner. They get a monthly/quarterly ad budget approved, run out to a high profile link broker, and buy a bunch of links, cross it off their to do list, and start playing angry birds.

Instead what you really should do is buy links slowly over a longer period time. Link buyers also put way to much focus on commercial keywords and destination URLs, without mixing them up and creating diversity. It’s incredibly easy to see, with tools available to you today.

Thats the gist of it, but I recommend reading the entire article here.