Common sense. Something apparently in short supply...
Black-hat SEO (and buying links is certainly black-hat SEO) is a huge risk. The search engines are constantly expanding their bag of tricks for discovering black-hat SEO.
The big risk is that if you get caught, it is more likely than not that you then have a permanent ban, or at least one that is going to take one heck of a lot of good karma to reverse.
You may not get caught today - but as evidenced by the threads here full of moaning about wave after wave of updates that catch more and more in the net - you WILL get caught tomorrow.