It would be great if the admin could add user levels, for example, visitor users and advertiser users, this could allow to sell annual subscriptions to the advertisers for an annual fee. Once the annual fee expires, the advertiser user should pay again to reactivate the ads.
The visitor user never would see the "insert free add" button, unles he pay the annual fee, and the button sould be "subscribe to post an add".
This user levels would allow to send special emails to the visitor users with news about the latest ads, promotionsm etc, and the advertiser users can recieve emails about renewing their accounts and special offers to allow the posting of new ads.