I want to build a twitter sort classifed engine

I want to create a twitter like classified engine where all the ads will be tweets u choose a category post picture and tweet it and people can search or browse through categories. It will not be a blogging portal nothing like twitter but just a twitter kind of classified engine. Friendly and cool. If you already have such script get back to me asap. As dont have time to build from scratch. Also with a script to invite your friend from google contact .. hotmail .. yahoo and twitter and facebook.