i think instead of having a forum set aside for paid advertising how about changing it this way:
Paid ads get the same flag as the "Important:" posts do on the other forums (they stay at the top) and each one gets a colour to differenciate it from the next. they also stay on top of every page. so when you change pages they go to the top of every page. Also you should make free ads automaticaly expire after 2 weeks and paid ads expire after a month. also get rid of the reply option all together, this will stop people from "bumping" their ads to the top, just let people delete the ad.