I found a few options for you:

have you checked out the
http://www.appthemes.com/blog/tag/classipress/
classipress its a plugin for wordpress

theres also

http://www.worksforweb.com/
http://open-classifieds.com/

From my understanding if you choose the open source software your going to have to hire a coder to modify it to look how you want it to look or function how you want it to function.