For some reason, people are under the impression that link buying is a SEO practice that one should not participate. I am not sure why some people think this way but they do. It might be because of all the chatter on reciprocal link exchanges, or building dozens of pages to build link popularity, or maybe all the news about how some SEO firms interlink all their clients and even worse, hide links from their client's sites to their own. So I can see why people might shy away from the link buying side of SEO.
A thread at HighRankings named How About Purchasing Text Links?, has a post by a relatively new member who asks the question; "Is it legitimate to purchase inbounds?" Now, HighRankings is known (IMO) to be one of the most "white hat" and conservative SEO forums out there (I may be wrong). The bottom line from the responses to this question in the forum are that there is nothing wrong with purchasing inbound links.