Contributors and readers of this thread might be interested to read the following (rather huge discussion) regarding buying and selling of links in attempts to increase Google page rank.
Matt Cutts: Gadgets, Google, and SEO » How to report paid links
It would seem that Google are already well on the way with their plans to attempt to put a stop to paid links affecting Page Rank. In concise terms, Google do not want page rank skewed by the selling and buying of links - their aim is to disregard links that have been bought/sold in terms of page rank. Therefore, buying links for that reason alone is probably ill advised. Buying links for traffic is another matter entirely. This stance from Google is not new, they have always advised that buying links solely in an attempt to increase page rank is not a strategy they like or condone. They have advised for some time that paid links should include a 'rel=nofollow' tag and it seems that they are moving ever closer towards ignoring paid links as a PR determining factor.
I picked this up from another thread here on the forums that steered me to a blog post about John Chow's fall from Google grace - many seem to feel because of paid links - that is just a rumour however and I don't personally believe it stacks up.
This has opened a can of worms alright, with recriminations and accusations flying everywhere, but the arguments from webmasters and Google themselves make for interesting reading and will maybe help you in your future SEO plans.
Google page rank, as Larwee has already pointed out in his earlier post, is not any kind of guarantee of a high search engine ranking for a particular keyphrase or keyword search.
Mo