I believe Google, especially Matt Cutts [mattcutts.com], care to fightback on link sellers. However, I think Google doesn't have the capacity or not as effective as it should be in that field.
Therefore I suggested previously that webmasters and sites owners co-operate with Google in reporting link sellers. That could be done within Sitemaps, the same way as reporting spam.
Haven't heard a word from Google about that suggestion, yet ;-)