As the owner of a small business, I can say with little doubt that selling advertising would be a difficult and thankless task. The national / worldwide nature of our lovely little forum means that we would have appeal to a small number of sellers and it would be a thankless task to be the person charged with the responsibility of recruiting the revenue.
A better option might be Google Adsense. Adsense allows the admin team to control where the ads go (even down to specific pages) and the ads chosen are driven by Google searches and matches with their own analysis tools. We get paid by Google for every ad that is displayed.
https://www.google.com/adsense/www/en_U ... index.html
I am on several forums where this became the only way to keep going and handled well it is not intrusive. I suspect I've never followed an ad but that doesn't stop the site getting paid.
There is also redtyger advertising for vBulletin websites but I know nothing about it.