If you are selling links - the worst penalty that you can get is loosing the ability to pass link juice.
If you are buying links - the worst thing for you is not getting PR you were paying for.
Google is not going to ban sites neither of seller, nor of buyer.
And remember two more things.
1. If site is shown in your backlinks, it doesn't mean that you get PR from it.
2. If Google was banning for buying links, if would be quite easy to ruin your competitor's business.