You're correct, selling links for pagerank is frowned heavily upon by Google. If you're caught then you can expect to get slapped with a pretty heavy pagerank penalty, at worst it can be zeroed which will impact heavily on your search rankings.
I buy and sell links all the time though, it's just a case of being a bit clever about how you do it. Avoid organised schemes at all costs, they're usually pretty obvious and it's easy for the whole network to get hit.
Also, link placement, footer links are almost always bought or paid in some way, and there's quite a bit of evidence to show that these are ignored or less effective.
Try to buy (or sell) editorial links, in relevant text, much harder to prove any non-editorial intent and far more effevtive in the long run.