Classified ad site needed

We are looking for a classified add site very very much like [login to view URL] with the easy to use? UI and posting engine, the captch style authentication systems, as well as the underlying systems administration capabilties. In additional to the features of CL and Kajiji that are already available we also? would like secure user authentication capabilties and the addition of upgraded service levels for paying members.? We want to allow developers to write to an API or WS interface? to allow development of autoposters and additional applications or enhancements from external sources. ? Additional enhancements to posted pictures in the paid subscription area are also required.