Buying Backlinks

If the links pass PageRank, Google would regard them as a breach of Webmaster Guidelines and may apply penalties. Bing feel much the same.

In short, yes, it should be avoided.

You can buy all the backlinks you like, and get banned all you like too! Search engines are cracking down on this stuff big time. Also, unless the content or domain itself that is linking to your content or domain is relevant to your content, then the backlink is completely worthless anyway, even if it legit.