I think it would be great if there was an addon that place banners saying "advertise here" or similar on the adspots that you desire to sell ads on. Users who wanna advertise just click the banner they wanna place their ad on and then being transferred to a form where they upload the banner, insert the link and then are requested to pay. When payment is successful admin should be able to choose accept or decline, if decline an refund would be issued.
Anyone that can make this (or something similar) happen? Searched but could'nt find anything like this.