I am looking for software which displays classifieds ads, preferably in PHP. It should also have the option of externally placed adverts like Google etc.

Which good ones exist out there?

I am also looking for an agency to handle adverts for the site.

I don't know how internet advertising business works so I will describe my needs .

In addition to the general or basic ads entered by everyday users, I want some parts of the site to be larger or specially placed adverts relating to the sites purpose.

Some too I want to be linked by ad agencies into the site, like Google Adsense etc. I might also consider directing those who want the better featured placements described above to be directed to the agency.

Are there any ad agencies like that, especially in the UK area?