Unless you clearly state in your website that you sell links, if you offer DO FOLLOW sponsored advertisement, etc than you might get in trouble. Lately I've heard that the SEs are also looking at forums and message boards for people selling links, so if you offer paid links try to maintain it as private as possible and don't publicly share in which website you are selling the links.
Very solid points being made. Thing I find strange is that some people will spend hundred even thousands of dollar buying links but won't spend a dime on building content that naturally gets them inbound links.