Paid links can work but you have to work with a webmaster that won't just label your link as a paid link. The search engine won't have a way of knowing the difference if the link isn't labeled. I sell links on my blog and I have a sitewide disclosure that some of my links are paid links. I haven't gotten in trouble with google thus far. I think as long as somewhere on the site you tell people that some links are paid links both parties will be okay.
You are just lucky that no one has ever reported your site of selling links. This is based on my experienced because I have been recently been penalized by Google because someone has reported my site.