Hi,
I went through your post here in detail and to summarize you need an app which would browse classifieds on the app , add classifieds from the app and have the possibility to chat / message other app users.
We have worked on apps where we have implemented all these features and there fore I am confident that we can deliver this app for you with ease.
I have posted some queries/comments in your PMB ,please take a quick look.
P.S. I could also get you in you in touch with a few of our American clients for a reference check if need be.
Cheers,
Rahul.