If buying links, avoid them.

It's so obvious to a Google manual reviewer some sidebar links, like the out of niche ones are paid links.

When a domain is 'caught' selling links by Google they no longer pass PR via external links.

I used to sell links and I've had a fair number of domains hit (as a seller I got a warnings via Google Webmaster Tools). As a business person I'm ethical, I stopped selling links from any domains hit with an unnatural links penalty, not all sellers are ethical.

Got to a point where it just wasn't worth the risk selling links, spend years building up decent PR domains only to have Google slap them for selling links the value of which (what you can sell them for) has dropped considerably over the past decade. It's one thing taking a risk to make thousands of dollars a year per domain selling links, but when it drops to hundreds it's not worth it (make more money other ways).

David

If it's a site whose traffic you don't mind losing when the next algorithm update comes around, you can buy thr sidebar links...and I can say with the utmost certainty that they are very effective but this largely depends on the authority of the site it's placed on.