We need Messaging system for buyers as well as sellers

Need a messaging system to be implemented as a module into shopping cart system. It would be great if to inactivate the messaging system didn't alter the cart function.
The message platform (with filter system) will be fiverr type for unlimited users (buyer, seller, and admin): consisting of "inbox, conversation, discussion" as well as "to do, list, activity, board".
The store (user name and password) governs the messaging system. The messaging system will be associated with related store function such as "ask about product", "email notification", "contact us" and alike. This will needs minimal code modification on shopping cart.
The system must fit current responsive web interface (skin), accessed through cell phone. (suitable for app).
We will define this project more in depth to the potential web programmer.
Must have experience with messaging system as well as e-commerce, better with xcart (modules), PHP.
If possible show your previous work. Y...

Desired Skills are AJAX, Joomla!, X-Cart, Messaging system, PHP MySQL