Is having a picture on the side, saying "Your ad here for $xx/month" considered as a link selling? An image points to: paypal.com/cgi-bin/webscr The site: healthhype.com (I'm not the owner and do not ask for ads)
[SIZE=-3]Put Your Advertisment Here [/SIZE]^^ Google kind of does that here on DP... However, it would likely depend on what the link looked like once it was up. Google requests that it be NOFOLLOW in some fashion (With rel=nofollow, or with a <script>).
It is considered link selling but as far as if Google punishes you for this no one really knows. I think Google is more punishing directories for selling links such as they go through phpdirectory scripts and punish them. I don't think google can really punish someone for selling links on their regular websites as it is advertising just like they do but like said above I would encourage that the advertising has no SEO value by putting a nofollow or using javascript.
If your sponsor link are nofollow as per google's guidelines there won't be any issue. But I have found google to be highly inaccurate in detecting sold links (but sometimes they are quick to detect sold links).
yes, it will be considered as link selling but if you use nofollow on sold links, you are fine with google
If you don't understand well about dofollow or nofollow, check out http://atniz.com/?tag=pagerank . It is my personal blog and I share the details I know about page rank and dofollow and nofollow issues. It is very easy once you know how to do it.