IMHO - Most people that talk about buying links, do it all wrong. The easier it is to buy the link, the easier it is for that paid link to turn into trouble, and the harder it will be to clean up the mess.
I regularly provide compensation in exchange for gaining links from relevant websites. I only target websites that my research leads me to think there is a good chance of generating click-through traffic that will lead to a conversion on my site. Sometimes I offer them content, prizes, gifts, monetary donations and similar items. All of these items cost me something, so I guess you could say I buy alot of paid links. I just stay away from public networks or websites that will sell links to anyone - they tend to be poor long term investments.