Lets discuss some of the aspects of buying links.

First, many link text brokerage sites sell links from a network of websites that may or may not have relevance to your own website. The problem here is one would be trading value for quantity. If a link is of no interest to visitors of this network of sites then the traffic from these sites will be very low. The only value of the link would be to boost page rank. But whether these links will boost page rank is debatable at best. Google penalizes many SEO experts believe that a link that is not relevant to the page from which the link is on. Still there are those who question this reasoning simply because Google bombing still works, as does blogspam. The topic is very debatable. Many claim that Google can't differentiate between a link that is paid for and one that is not. Logically this is correct, but Google can spider the contents of a page, and if the links on that page are of no relevance to the rest of the content the spider knows. Of course there is no direct evidence that Google will penalize your link if it is on a page that has no relevance to your link. It is merely speculation. But as I said, even though the spider doesn't know if you paid for the link or not, they are made to spider content. This thought should cause you to proceed with caution when and if you choose to buy links. Always evaluate your intent when buying links. Is it just to boost page rank, or will the link also be of benefit to the visitors of the page the link will be placed on. Both are important to the level of traffic the link could bring.