Is it against SEO guidelines to offer a free advertisement on my website?

[Roman Iakoubtchik]

As far as I know there are no universal SEO guidelines. Google for instance requires that all advertising are clearly marked and contain the rel="nofollow" link attribute.

Google's explanation: "A site's ranking in Google search results is partly based on analysis of those sites that link to it. In order to prevent paid links from influencing search results and negatively impacting users, we urge webmasters use nofollow on such links. Search engine guidelines require machine-readable disclosure of paid links in the same way that consumers online and offline appreciate disclosure of paid relationships (for example, a full-page newspaper ad may be headed by the word "Advertisement")."