I think this will be safe for both the site owners and the link buyers who just need only human visitors' clicks (as same as PPC). The tag rel="nofollow" is OK for the webmaster guidelines. What do you think? Thanks.
These days, nofollow is also considered in SEO...as search engine is able to know where you are getting nofollow links from, where you are getting traffic from.. It is better today if you build brand name, I would say!