When it comes to buying links, be wary. Google has announced that it will not count paid links. What that means is, if it is obvious that a link is a paid link then it is likely that Google will discount it. That’s not necessarily a penalty. You won’t lose PageRank for buying links, but if you pay for a link and Google doesn’t count it then you have effectively thrown your money out the window. In linking strategies, Google is the toughest of all the search engines but you can bet that what Google implements into its algorithms will eventually be copied by the other players, so watch out.
A better strategy is to purchase a link for its traffic value. This is better for you anyway because what you really want to develop is targeted traffic. PageRank and other ratings (such as Alexa) are merely measurements and they can be misleading. You can have a highly ranked website that doesn’t get enough targeted traffic or that doesn’t convert the traffic that it gets. If you’re in that boat then you need to work on those deficiencies. If your PageRank drops and your targeted traffic increases, you have helped yourself more than you ever could.
So when, and if, you should decide that buying links is something you want to do, look for websites that show they can deliver you targeted traffic. Those are the best kinds of links anyway.