When you have things of value that can be sold or exchanged you will allways have a market. If some authority tries to ban the exchange then it will be a 'black' market. The harder Google tries to enforce their policies the more it will drive the selling or exchange of links underground. Maybe this would be a good thing but there is certainly no way they can stamp it out.
I personally think that the problem lies in the rules of Google's system. A more robust system should be able to account for and adjust for the market realities created by it's own rules. Maybe it is technically impossible but I doubt it. I'm sure the clever people at google have thought of this as well but perhaps they are starting to face 'legacy' system issues themselves.
Google prefers developing scalable and automated solutions to problems, so we attempt to minimize hand-to-hand spam fighting. The spam reports we receive are used to create scalable algorithms that recognize and block future spam attempts.
I'll bet they do but they can only detect the most obvious cases. It would be possible for website to sell links that even a human couldn't identify as sold, so there are limits to what their algorithms could achieve.
I personally think that the problem lies in the rules of Google's system. A more robust system should be able to account for and adjust for the market realities created by it's own rules. Maybe it is technically impossible but I doubt it. I'm sure the clever people at google have thought of this as well but perhaps they are starting to face 'legacy' system issues themselves.