Google's attempt to stop link selling won't work in my opinion. They will just drive link selling underground - similar to a 'black market'. Manually reducing the PR of some of the most visible sites that sell PR just opens up the playing field for newer websites to sell their PageRank now that the well-known PageRank'ed sites got a downgrade. In addition, many people purchase links not just for the PageRank but also for the "anchor text" link to rank for specific terms, and for an additional backlink to their site for more authority! So there are still alot of other reasons why the "Buying and Selling Links" industry will still flourish. The only definitive way for Google too stop this whole thing is to remove PageRank from their toolbar (which of course they don't to do because this is the only reason most webmasters install their Toolbar and Google does not want them to uninstall it), and stop giving out information about the number of backlinks a site has. As far as anchor text links, people already know this is a major part of getting ranked for keywords in Google's algorithm and I see now way in the forseeable future that Google can resolve this unless they take this out of their algorithm as a factor for ranking sites in their SERP's. Just my 2 cents from a nobody.
Either that or there will be a new accepted metric for determining the value of a text link. The thing is, Google not only lowered the PR, they most likely inhibited the ability of those links to pass PR as well
Google should just ditch PR (not just from showing in the toolbar but from ever being publicly accessible). It's nothing but a webmaster's obsession and a meaningless number. A value of a page is the quality of content and how big an audience it reaches (or can potentially reach). The time spent obsessing about PR could be better spent on more research when writing content and more care taken in writing it. Additionally, site accessibility - easily navigable, quick to load, etc. Let's remember that we are after all making content for people to read/interact with and they will only find/return if it's good. Google should its algorithm to focus more on valuing a page for its content and less on links/anchor text if it's so concerned about link-selling. (As the OP wrote, it won't stop link selling, just drive it underground.) Just my 2 cents.