How about creating extra options for existing forum structure to turn them easily into classifieds.
Lets say...admin want to create classified section on his forum..
admin side settings could include:
1) option to delete ads (posts) after X days [idea]
2) payment option (PayPal) for highlighting certain ad (thread) on thread list (for X amount of days)
3) payment option (PayPal) to keep them on the top for X days (like current sticky - but named & marked maybe premium ads) [idea]
4) custom / required fields like : PRICE, CONDITION, ZIPCODE, ad type (for sale/trade/free)
5) placement for Adsense
user side settings could work this way:
1) typed in price will show up in bold on thread list (maybe in place of REPLIES# or below thread title)
2) picture upload (same as current attachments).. 1st uploaded picture could show up on theadlist next or instead of user avatar.
3) map or distance calculation based on seller zipcode (google maps API)
just to get the idea, example of similar solution:
What do you guys think?