fantastic classifieds website

I am looking to build next generation classifieds platform where people can post classifieds on my website for free, they can either join by fb connect or normal sign up and can post ads in as many categories as possible something on the same lines of olx but better ... have you created anything before on the same lines please show me your work. Remember I am not looking to do it as custom development. I need already done work and I will try and give it a personal touch.