How to build desktop application to post ads in internet automatically

I haven't yet created an "out-of-browser" silverlight application but I think its a good start for you. You can detached a silverlight web-base application to a browser making it like a desktop application that has access to a web server. If I'm not wrong, use a "web services" that silverlight supports and that the web application (that the internet classified website) you want to hook-up must have an exposed method (using web service) that your application can consume.

Try also to research on using web service in desktop application. This might also help you.

Sorry if I can't provide you a code.