Dear Friends.. Is there any site for submitting classifieds as socialposter.com is there for submitting bookmarks in Bookmarks site.
I would suggest you to spend some time to make a list of quality classified websites and to handle submissions by yourself. Using automated submission forms would create a big mess you wouldn't be able to clean up.
Manual submissions should always be preferred. Websites like social posters would take you to predefined websites and would not take into consideration the theme of your website. It would recommended if you research and find classifieds that are relevant to your website. Craigslist is still the best and can give you lots of traffic.