Buying text links…
There are quite a few marketplace-type sites out there facilitating text link trading. I don’t mean link exchanges, I mean buying and selling text links.
I’ve read a few reports of high profile sites having their PageRank-passing-on-ability removed by Google because of this, but for the most part it’s probably safe. The main reason being so many sites are doing it that the skew caused by removing those sites could well be bigger than the skew caused by leaving them in. On the other hand, it may not be that much of a problem. Although it is a very suspect argument, you could say that a site willing to pay that much (and we’re talking upwards of $15 per month per link) must have something to offer.
At the moment it doesn’t always work like that since many sites are some sort of affiliate adding no new content or are domain-parking-like sites. People are becoming aware of these things though (it’s easy to spot such pages in Google’s own sponsored sites section of search results) and as such more useful sites are actually using this as a valid advertising source.
If you’re interested you might try Text Link Brokers or Linkworth. Linkworth have lots of little tools people can use without actually buying or selling any links and have no minimum PR, and are more open. Text Link Brokers have been at it for longer and have more high PR sites (some PR 8 sites with .gov back-links).