Do you think that google have his own ways for detecting people who sell links (including in forums) and do you think they punish the websites that are selling links?!
Well, Matt said Google can take action to return the best search results and webmasters can do whatever they want for their sites.
If there is a text on the site: "selling links for...$" then Google can find this, can't it? But beyond that, Google may only assume that someone bought links - like got it a lot in a short time, or all links have the same anchor text.
o better then all put an 50 years old women to watch forums how many people ask link details in dp?! and how many of them really buy the links?!
Not sure if Google can spy PMs. The point is how Google can obtain good results. Some women should rather check SERPs for quality sites. But kicking PR out of their algo can do a lot.
I think they do most of the tasks by their algo, but sometimes alghorithm is supported by people's hands.
Google can detect paid content listing if it done explicitly through paid linking services ( like PayPerPost ).