They way I understand it is that when a person puts a link on their cache page to a site, it can not be a commercial site. If the person puts a link to
www.geocachingmaine.org, than that initial page can not have google ads. The rest of the website is okay to have the google ads as well as a buy and sell forum.
This is the impression that I got from our reviewer, however, that was before the google ad addition on the cache listing pages, so maybe things have changed.
The issue comes up when the link is done in the other direction, from gc.com to a website.