Dear sir.
I have just checked out Carousell app you need.
Carousell app include the following functions :
1. user registeration ( option : with Facebook, Twitter etc ) and login
2. Take photo of user' selling product and manipulation ( apply with color effect and scale etc )
3. uploda photo to server( option : share to Facebook, twitter etc )
4. search the user with the sharing photo ( selling photo) based on the search location
5. chat between buyer and seller
6. manage user's registration information
7. manage app setting information
***** I checked out there is nothing the payment system in this app. *****
I think your project should be run with three system : server system, iOS app and android app.
I'd like to add video chat. I prefer VoIP system like Linphone. If you need your own VoIP system, it will be need the server system supported VoIP like Asterisk.
Also your project need the functions for the management of users(buyer and seller) and the data including photos and chat history etc.
In a word, your project is so interesting and I'd like to get the chance for completing your project from you. I am a small software team leader and I have server/web app developers and iOS/android app developers. As you can see on the above description, I have my plan for completing your project and I have top skill and rich experience for it.
I'd like to see you and discuss on Skype.
Regards